refactor(#2441): move agent field from DAG onto Node; drop dedup

Agent was a single field on Dag/DagSpec, making a DAG structurally
one-agent — a multi-agent op could only ever be N separate DAGs. Move it
onto Node/NodeSpec (and the NodeView wire type), drop it from Dag/DagSpec
(and DagView): a DAG can now span agents.

- lifecycle lease keys on the node's agent, still globally exclusive per
  agent across all DAGs (Inner.leases unchanged in shape). A DAG holds one
  lease per distinct agent it touches; settle() frees each at DAG-terminal
  (per-agent-subgraph early release is a follow-up, only observable with
  multi-agent DAGs).
- transient guard keyed (dag_id, agent); cancel-revert + Rebuilt events
  walk TerminalDag.agents.
- submit-time dedup removed (a multi-agent DAG has no single agent to key
  on); every submit enqueues a fresh DAG. Whether dedup needs reintroducing
  is tracked in a follow-up sub-issue.
- templates gain a node(agent, kind, deps) helper stamping the agent onto
  every node; meta templates stamp "hyperhive".

Templates stay single-agent in this PR — behaviour is unchanged, only the
representation + wire shape. Multi-agent DAG emission (restart/restart-all/
broad stop+start as one DAG) and the SetWanted-as-a-node change are
follow-ups off #2439.

hive-sh4re/src/jobs.rs

@@ -150,6 +150,11 @@ pub type NodeId = u32;
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct NodeView {
     pub id: NodeId,
+    /// The agent whose container (or meta repo, for `hyperhive` meta-level
+    /// nodes) this node operates on. Agent is per-node — a single DAG can
+    /// span multiple agents (e.g. a hive-wide restart), so there is no
+    /// DAG-level agent field; consumers group by this.
+    pub agent: String,
     /// Node primitive tag: `"prebuild"`, `"stop_for_update"`,
     /// `"swap"`, `"create"`, `"meta_lock"`, `"reconcile"`, `"signal"`,
     /// `"drain"`, `"write_dropin"`, `"write_perm_file"`,
@@ -171,13 +176,13 @@ pub struct NodeView {
     pub error: Option<String>,
 }
 
-/// A queued/running/recent DAG. DAG-level fields mirror the pre-DAG
-/// `QueueEntry` names (`kind` = template string, roll-up `state`);
-/// everything per-node appears exactly once, inside `nodes`.
+/// A queued/running/recent DAG. `kind` = template string, roll-up
+/// `state`; everything per-node appears exactly once, inside `nodes`.
+/// There is no DAG-level `agent` — a DAG can span agents, so agent lives
+/// on each [`NodeView`]; consumers group nodes by `NodeView::agent`.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct DagView {
     pub id: u64,
-    pub agent: String,
     /// Template wire string — same values the old `kind` field used.
     pub kind: Template,
     /// Roll-up: `failed` if any node failed, else `running` /