feat(#2398): dagify hivectl restart (RestartScoped)

hivectl restart --agent NAME previously composed stop() then start() as
two separate client-side daemon calls glued by CLI-process control flow
— not one DAG, and a dropped hivectl connection mid-restart (ssh drop,
Ctrl-C) left the agent stopped with no automatic follow-up. mara flagged
this as the first target for the 'dagify hivectl commands' issue.

New HostRequest::RestartScoped{scope, graceful} handles it server-side:
each targeted agent now rides exactly one atomic Restart-template DAG
(same one hivectl agents restart / restart-all already use) in the
common non-graceful case. --graceful has no single-DAG template yet, so
it submits the graceful-stop DAGs, awaits them server-side, then submits
the start DAGs — still one daemon call end to end, just not yet a single
DAG (noted as a follow-up). Infra containers restart synchronously as
before (no lease/DAG concept for them).

CLI-side restart() now just makes the one call + waits, same output
shape as before via render_lifecycle.

hive-host-sock/src/lib.rs

@@ -39,6 +39,23 @@ pub enum HostRequest {
     /// wrapper for `hivectl agents restart-all`; iterates the live
     /// container list and restarts each one.
     RestartAll,
+    /// Restart containers hive-wide (`hivectl restart`), scoped like
+    /// `Stop`/`Start`. Each targeted agent rides exactly one DAG server-side
+    /// (the `Restart` template, hard stop + reconcile — or, when `graceful`
+    /// is set, a submitted graceful-stop DAG the server itself waits out
+    /// before submitting the start DAG) rather than the old client-side
+    /// stop-then-start composition: a dropped `hivectl` connection mid-way
+    /// used to leave the agent stopped with no automatic follow-up, since
+    /// nothing durable remembered "finish the restart" once the CLI process
+    /// was gone. Infra containers have no lease/DAG and restart
+    /// synchronously (stop then start), same as before. Scope semantics
+    /// match `Stop`/`Start` (all-false = everything).
+    RestartScoped {
+        #[serde(default)]
+        scope: LifecycleScope,
+        #[serde(default)]
+        graceful: bool,
+    },
     /// Apply pending config to a managed container.
     Rebuild { name: String },
     /// List managed containers.