test+docs(#2448): multi-agent restart test + doc shapes

- multi_agent_restart_is_one_dag_with_concurrent_per_agent_subgraphs:
  two agents restart in one DAG; both SetWanted heads are roots, each
  acquiring its own agent's lease concurrently. Existing single-agent
  shape/lease tests updated for the templates::restart(&[..], graceful)
  signature.
- coordinator.md + templates.rs module doc: restart takes an agent list;
  a hive-wide restart is one DAG with N per-agent subgraphs.

docs/coordinator.md

@@ -76,12 +76,15 @@ container build:
 
 The power ops write the durable `wanted` intent via a head `SetWanted`
 node (not a pre-submit side effect) — it holds the agent lease, so
-intent-write + reconcile is atomic per-agent.
+intent-write + reconcile is atomic per-agent. `restart` takes an agent
+*list*: a hive-wide `hivectl restart` / `restart-all` is ONE DAG with a
+per-agent restart subgraph each (independent roots, run concurrently on
+their own leases), not N separate DAGs.
 
 ```text
 rebuild(a):        Prebuild(a) → StopForUpdate(a) → Swap(a) →(after-any) Reconcile(a)
 graceful-stop(a):  SetWanted(a,Off) → Signal(a) → Drain(a) → Reconcile(a)
-restart(a):        SetWanted(a,Up) → StopForUpdate(a) → Reconcile(a)
+restart(a..):      per agent: SetWanted(a,Up) → StopForUpdate(a) → Reconcile(a)   (N subgraphs, 1 DAG)
 graceful-restart(a): SetWanted(a,Up) → Signal(a) → Drain(a) → StopForUpdate(a) → Reconcile(a)
 start(a):          SetWanted(a,Up) → Reconcile(a)     (stale rev ⇒ stale-start below)
 stale-start(a):    SetWanted(a,Up) → «rebuild subgraph»  (prebuild noops — agent is down)

hive-c0re/src/job_queue/submit.rs

@@ -27,12 +27,27 @@ pub fn rebuild(coord: &Arc<Coordinator>, agent: &str, source: Source, reason: St
     submit_and_emit(coord, templates::rebuild(agent, source, reason, None, true))
 }
 
-/// Restart: mechanical stop + converge to `wanted = Up`. The intent
-/// write is the template's head `SetWanted(Up)` node — it matters when
-/// `wanted` drifted `Offline` under a running agent (an operator asking
-/// for a restart plainly wants it running, not a stop).
+/// Restart a single agent: mechanical stop + converge to `wanted = Up`.
+/// The intent write is the template's head `SetWanted(Up)` node — it
+/// matters when `wanted` drifted `Offline` under a running agent (an
+/// operator asking for a restart plainly wants it running, not a stop).
+/// Thin wrapper over [`restart_many`] with a one-agent slice.
 pub fn restart(coord: &Arc<Coordinator>, agent: &str, source: Source, reason: String) -> u64 {
-    submit_and_emit(coord, templates::restart(agent, source, reason))
+    restart_many(coord, &[agent.to_owned()], false, source, reason)
+}
+
+/// Restart `agents` (one or many) in a **single** DAG — one per-agent
+/// subgraph each, running concurrently on their own leases. `graceful`
+/// prepends the signal→drain quiesce per agent. The whole hive-wide
+/// `hivectl restart` / `restart-all` is now one DAG instead of N.
+pub fn restart_many(
+    coord: &Arc<Coordinator>,
+    agents: &[String],
+    graceful: bool,
+    source: Source,
+    reason: String,
+) -> u64 {
+    submit_and_emit(coord, templates::restart(agents, graceful, source, reason))
 }
 
 /// Start: `SetWanted(Up)` (a DAG node now) then reconcile. A stale rev
@@ -67,17 +82,18 @@ pub fn graceful_stop(coord: &Arc<Coordinator>, agent: &str, source: Source, reas
     submit_and_emit(coord, templates::graceful_stop(agent, source, reason))
 }
 
-/// Graceful restart: signal → drain → mechanical stop → reconcile (starts
-/// it back up) — one atomic DAG, no client-side "await the stop DAG then
-/// submit a start DAG" split. The head `SetWanted(Up)` node writes the
-/// intent as part of the DAG.
+/// Graceful restart of a single agent: signal → drain → mechanical stop →
+/// reconcile (starts it back up) — one atomic DAG, no client-side "await
+/// the stop DAG then submit a start DAG" split. The head `SetWanted(Up)`
+/// node writes the intent as part of the DAG. Thin wrapper over
+/// [`restart_many`] with `graceful = true`.
 pub fn graceful_restart(
     coord: &Arc<Coordinator>,
     agent: &str,
     source: Source,
     reason: String,
 ) -> u64 {
-    submit_and_emit(coord, templates::graceful_restart(agent, source, reason))
+    restart_many(coord, &[agent.to_owned()], true, source, reason)
 }
 
 /// Perm change: commit the JSON file(s) then rebuild.

hive-c0re/src/job_queue/templates.rs

@@ -4,9 +4,11 @@
 //! `Vec<Node>` + `deps`).
 //!
 //! Every node carries its own `agent` (there is no DAG-level agent) — the
-//! `node` helper stamps the template's agent onto each. Today's templates
-//! are single-agent (every node shares one agent); a future multi-agent
-//! template would stamp different agents per subgraph.
+//! `node` helper stamps the template's agent onto each. `restart` takes an
+//! agent *list* and stamps each agent onto its own subgraph, so a hive-wide
+//! `hivectl restart` is ONE DAG with N independent per-agent subgraphs
+//! (each a root chain, run concurrently on its own lease) rather than N
+//! separate DAGs. The other templates are still single-agent.
 //!
 //! The power ops write the durable `wanted` intent via a head
 //! `SetWanted(w)` node (not a pre-submit side effect); it holds the agent
@@ -146,37 +148,36 @@ pub fn graceful_stop(agent: &str, source: Source, reason: String) -> DagSpec {
     }
 }
 
-/// Restart: write `wanted = Up` (head `SetWanted` node), mechanical stop,
-/// then converge — a stop + start like the old `lifecycle::restart`
-/// regardless of prior intent drift.
-pub fn restart(agent: &str, source: Source, reason: String) -> DagSpec {
-    DagSpec {
-        template: Template::Restart,
-        source,
-        reason,
-        parent_id: None,
-        approval_id: None,
-        inputs: Vec::new(),
-        perm_payload: None,
-        transient: Some(TransientKind::Restarting),
-        nodes: vec![
-            node(agent, NodeKind::SetWanted { up: true }, Vec::new()),
-            node(agent, NodeKind::StopForUpdate, after_ok(0)),
-            node(agent, NodeKind::Reconcile, after_ok(1)),
-        ],
+/// Restart one or more agents in a **single** DAG. Each agent gets an
+/// independent subgraph — a head `SetWanted(Up)` (a root: no cross-agent
+/// dep) then its restart chain — so all agents' restarts run concurrently,
+/// each taking its own agent lease. `graceful` inserts `Signal → Drain`
+/// before the mechanical `StopForUpdate` (each agent's subgraph is still one
+/// atomic restart). One agent = the ordinary single-agent restart; many =
+/// a hive-wide `hivectl restart` as one DAG instead of N separate ones.
+pub fn restart(agents: &[String], graceful: bool, source: Source, reason: String) -> DagSpec {
+    let mut nodes = Vec::new();
+    for agent in agents {
+        let base = u32::try_from(nodes.len()).unwrap_or(u32::MAX);
+        // Head of this agent's subgraph — a root (empty deps), so the N
+        // per-agent subgraphs are independent and run concurrently.
+        nodes.push(node(agent, NodeKind::SetWanted { up: true }, Vec::new()));
+        if graceful {
+            nodes.push(node(agent, NodeKind::Signal, after_ok(base)));
+            nodes.push(node(agent, NodeKind::Drain, after_ok(base + 1)));
+            nodes.push(node(agent, NodeKind::StopForUpdate, after_ok(base + 2)));
+            nodes.push(node(agent, NodeKind::Reconcile, after_ok(base + 3)));
+        } else {
+            nodes.push(node(agent, NodeKind::StopForUpdate, after_ok(base)));
+            nodes.push(node(agent, NodeKind::Reconcile, after_ok(base + 1)));
+        }
     }
-}
-
-/// Graceful restart: write `wanted = Up` (head `SetWanted`), signal →
-/// drain → mechanical stop → converge. One atomic DAG start to finish
-/// (no client- or server-side "submit one DAG, await it, submit the next"
-/// composition): the `Drain` node is the same bounded harness-checkpoint
-/// wait `graceful_stop` uses, then `StopForUpdate` (mechanical, ignores
-/// `wanted`) and the tail `Reconcile` (converges to `wanted = Up`, i.e.
-/// starts it back up) chain exactly like `restart`'s tail.
-pub fn graceful_restart(agent: &str, source: Source, reason: String) -> DagSpec {
     DagSpec {
-        template: Template::GracefulRestart,
+        template: if graceful {
+            Template::GracefulRestart
+        } else {
+            Template::Restart
+        },
         source,
         reason,
         parent_id: None,
@@ -184,13 +185,7 @@ pub fn graceful_restart(agent: &str, source: Source, reason: String) -> DagSpec
         inputs: Vec::new(),
         perm_payload: None,
         transient: Some(TransientKind::Restarting),
-        nodes: vec![
-            node(agent, NodeKind::SetWanted { up: true }, Vec::new()),
-            node(agent, NodeKind::Signal, after_ok(0)),
-            node(agent, NodeKind::Drain, after_ok(1)),
-            node(agent, NodeKind::StopForUpdate, after_ok(2)),
-            node(agent, NodeKind::Reconcile, after_ok(3)),
-        ],
+        nodes,
     }
 }
 

hive-c0re/src/job_queue/tests.rs

@@ -67,7 +67,12 @@ fn distinct_submits_never_collapse() {
     let b = submit(&q, rebuild("agent-b", "r"));
     let c = submit(
         &q,
-        templates::restart("agent-a", Source::Manual, "r".to_owned()),
+        templates::restart(
+            &["agent-a".to_owned()],
+            false,
+            Source::Manual,
+            "r".to_owned(),
+        ),
     );
     assert_ne!(a, b);
     assert_ne!(a, c);
@@ -200,7 +205,12 @@ fn lease_serializes_two_lifecycle_dags_for_same_agent() {
     let q = JobQueue::new(4);
     let restart = submit(
         &q,
-        templates::restart("agent-a", Source::Manual, "restart".to_owned()),
+        templates::restart(
+            &["agent-a".to_owned()],
+            false,
+            Source::Manual,
+            "restart".to_owned(),
+        ),
     );
     let stop = submit(
         &q,
@@ -284,16 +294,60 @@ fn agents_do_not_contend_on_each_others_leases() {
     let q = JobQueue::new(4);
     submit(
         &q,
-        templates::restart("agent-a", Source::Manual, "r".to_owned()),
+        templates::restart(
+            &["agent-a".to_owned()],
+            false,
+            Source::Manual,
+            "r".to_owned(),
+        ),
     );
     submit(
         &q,
-        templates::restart("agent-b", Source::Manual, "r".to_owned()),
+        templates::restart(
+            &["agent-b".to_owned()],
+            false,
+            Source::Manual,
+            "r".to_owned(),
+        ),
     );
     let claims = q.claim_ready();
     assert_eq!(claims.len(), 2, "different agents run concurrently");
 }
 
+#[test]
+fn multi_agent_restart_is_one_dag_with_concurrent_per_agent_subgraphs() {
+    let q = JobQueue::new(4);
+    let id = submit(
+        &q,
+        templates::restart(
+            &["agent-a".to_owned(), "agent-b".to_owned()],
+            false,
+            Source::Manual,
+            "hive-wide".to_owned(),
+        ),
+    );
+    // A hive-wide restart is ONE DAG, not one-per-agent.
+    assert_eq!(q.snapshot().len(), 1);
+    // Each agent's subgraph head (SetWanted) is a root, so both are
+    // claimable at once — each takes its OWN agent's lease (no contention
+    // across distinct agents), all inside the single DAG.
+    let claims = q.claim_ready();
+    assert!(claims.iter().all(|c| c.dag_id == id));
+    let mut heads: Vec<(&str, &str, bool)> = claims
+        .iter()
+        .map(|c| (c.agent.as_str(), c.kind.as_str(), c.lease_acquired))
+        .collect();
+    heads.sort_unstable();
+    assert_eq!(
+        heads,
+        vec![
+            ("agent-a", "set_wanted", true),
+            ("agent-b", "set_wanted", true),
+        ],
+        "both per-agent subgraphs start concurrently, each acquiring its own lease"
+    );
+}
+
 // ---- failure: cancel-downstream + AfterAny ----
 
 #[test]
@@ -514,7 +568,12 @@ fn terminal_dag_reported_exactly_once_and_lease_released() {
     let q = JobQueue::new(1);
     let id = submit(
         &q,
-        templates::restart("agent-a", Source::Manual, "r".to_owned()),
+        templates::restart(
+            &["agent-a".to_owned()],
+            false,
+            Source::Manual,
+            "r".to_owned(),
+        ),
     );
     // restart = SetWanted → StopForUpdate → Reconcile; not terminal until
     // the last node completes.
@@ -622,7 +681,12 @@ fn trim_keeps_terminal_parent_with_live_children() {
     );
     let lock = claim_one(&q);
     q.complete_node(meta, lock.node_id, Ok(()));
-    let mut child_spec = templates::restart("agent-x", Source::MetaUpdate, "cascade".to_owned());
+    let mut child_spec = templates::restart(
+        &["agent-x".to_owned()],
+        false,
+        Source::MetaUpdate,
+        "cascade".to_owned(),
+    );
     child_spec.parent_id = Some(meta);
     let child = submit(&q, child_spec);
     // Churn > MAX_HISTORY_PER_TEMPLATE terminal meta_update DAGs.

hive-c0re/src/server.rs

@@ -552,28 +552,29 @@ fn submit_single(coord: &Arc<Coordinator>, name: &str, verb: Verb) -> HostRespon
     HostResponse::queued(vec![id])
 }
 
-/// Restart every container by submitting one restart DAG per agent —
-/// each serializes on its own lease, so unrelated agents' restarts
-/// overlap while nothing races an in-flight rebuild. Returns once all
-/// are queued; per-agent results surface on the queue.
+/// Restart every container in **one** DAG — a per-agent restart subgraph
+/// each, running concurrently on their own leases (so unrelated agents'
+/// restarts overlap while nothing races an in-flight rebuild). Returns
+/// once queued; per-node progress surfaces on the single DAG.
 async fn handle_restart_all(coord: &Arc<Coordinator>) -> Result<HostResponse> {
     tracing::info!("restart-all");
-    let agents = lifecycle::list().await?;
-    let mut ok_agents: Vec<String> = Vec::new();
-    let mut queued: Vec<u64> = Vec::new();
-    for agent in &agents {
-        let Some(logical) = agent.strip_prefix(lifecycle::AGENT_PREFIX) else {
-            continue;
-        };
-        queued.push(crate::job_queue::submit::restart(
+    let containers = lifecycle::list().await?;
+    let agents: Vec<String> = containers
+        .iter()
+        .filter_map(|a| a.strip_prefix(lifecycle::AGENT_PREFIX).map(str::to_owned))
+        .collect();
+    let queued = if agents.is_empty() {
+        Vec::new()
+    } else {
+        vec![crate::job_queue::submit::restart_many(
             coord,
-            logical,
+            &agents,
+            false,
             crate::job_queue::Source::Manual,
             "manual restart via hivectl restart-all".to_owned(),
-        ));
-        ok_agents.push(logical.to_owned());
-    }
-    let mut resp = HostResponse::list(ok_agents);
+        )]
+    };
+    let mut resp = HostResponse::list(agents);
     resp.queued_dags = Some(queued);
     Ok(resp)
 }
@@ -718,16 +719,14 @@ async fn handle_start(
 }
 
 /// Restart containers hive-wide (`hivectl restart`) — the DAG-based
-/// sibling of [`handle_stop`]/[`handle_start`], replacing the old
-/// client-side stop-then-start composition (issue tracker "dagify hivectl
-/// commands"). Each targeted agent gets exactly one atomic DAG submitted
-/// up front: the `Restart` template (mechanical stop + reconcile) in the
-/// common case, or `GracefulRestart` (signal → drain → mechanical stop →
-/// reconcile) with `graceful` set — no "submit a DAG, wait for it, submit
-/// another" composition on either path, so a dropped `hivectl` connection
-/// never strands an agent, the same way `handle_restart_all` already
-/// avoids it. Infra containers have no lease/DAG and restart
-/// synchronously (stop then start).
+/// sibling of [`handle_stop`]/[`handle_start`]. All targeted agents ride
+/// **one** DAG (a per-agent restart subgraph each: `SetWanted → [Signal →
+/// Drain →] StopForUpdate → Reconcile`, independent roots that run
+/// concurrently on their own leases), not N separate DAGs — a hive-wide
+/// restart is one job. `graceful` prepends signal→drain per agent. No
+/// client-side stop-then-start composition, so a dropped `hivectl`
+/// connection never strands an agent. Infra containers have no lease/DAG
+/// and restart synchronously (stop then start).
 async fn handle_restart_scoped(
     coord: &Arc<Coordinator>,
     scope: &LifecycleScope,
@@ -740,30 +739,25 @@ async fn handle_restart_scoped(
     let mut errors: Vec<String> = Vec::new();
     let mut queued: Vec<u64> = Vec::new();
 
-    // One atomic DAG per agent, submitted up front — `Restart`
-    // (mechanical stop + reconcile) or, with `--graceful`,
-    // `GracefulRestart` (signal → drain → mechanical stop → reconcile).
-    // No client- or server-side "submit a stop DAG, await it, then
-    // submit a start DAG" composition: that window is exactly the
-    // dropped-connection gap this DAG-based path exists to close.
-    for agent in &agents {
-        let id = if graceful {
-            crate::job_queue::submit::graceful_restart(
-                coord,
-                agent,
-                crate::job_queue::Source::Manual,
-                "manual via hivectl restart --graceful".to_owned(),
-            )
-        } else {
-            crate::job_queue::submit::restart(
-                coord,
-                agent,
-                crate::job_queue::Source::Manual,
-                "manual restart via hivectl restart".to_owned(),
-            )
-        };
-        queued.push(id);
-        ok_items.push(agent.clone());
+    // One DAG for all targeted agents — a per-agent restart subgraph each
+    // (`SetWanted → [Signal → Drain →] StopForUpdate → Reconcile`),
+    // independent roots that run concurrently on their own leases. A
+    // hive-wide `hivectl restart` is now a single DAG, not N. No
+    // client-side stop-then-start composition — the whole restart survives
+    // a dropped connection because the DAG owns it.
+    if !agents.is_empty() {
+        queued.push(crate::job_queue::submit::restart_many(
+            coord,
+            &agents,
+            graceful,
+            crate::job_queue::Source::Manual,
+            if graceful {
+                "manual via hivectl restart --graceful".to_owned()
+            } else {
+                "manual restart via hivectl restart".to_owned()
+            },
+        ));
+        ok_items.extend(agents.iter().cloned());
     }
 
     for &container in &infra {