fix(#2398): graceful restart as one atomic DAG, not compose-and-await

mara's review on #2436: no submit-await-submit composition, even
server-side. Adds Template::GracefulRestart (Signal -> Drain ->
StopForUpdate -> Reconcile, wanted=Up) mirroring how Restart already
does StopForUpdate -> Reconcile, plus submit::graceful_restart and
templates::graceful_restart. handle_restart_scoped now submits exactly
one DAG per agent up front for both the graceful and non-graceful
case -- no await_dags in the loop anymore.

hive-c0re/src/job_queue/submit.rs

@@ -100,6 +100,19 @@ pub fn graceful_stop(coord: &Arc<Coordinator>, agent: &str, source: Source, reas
     submit_and_emit(coord, templates::graceful_stop(agent, source, reason))
 }
 
+/// Graceful restart: persist `wanted = Up`, then 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.
+pub fn graceful_restart(
+    coord: &Arc<Coordinator>,
+    agent: &str,
+    source: Source,
+    reason: String,
+) -> u64 {
+    set_wanted(coord, agent, Wanted::Up);
+    submit_and_emit(coord, templates::graceful_restart(agent, source, reason))
+}
+
 /// Perm change: commit the JSON file(s) then rebuild.
 pub fn perm_change(
     coord: &Arc<Coordinator>,