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/exec.rs

@@ -507,7 +507,11 @@ pub(super) async fn on_dag_terminal(coord: &Arc<Coordinator>, terminal: &Termina
     if terminal.state == State::Cancelled
         && matches!(
             terminal.template,
-            Template::Start | Template::Stop | Template::GracefulStop | Template::Restart
+            Template::Start
+                | Template::Stop
+                | Template::GracefulStop
+                | Template::Restart
+                | Template::GracefulRestart
         )
     {
         let running = crate::lifecycle::is_running(&terminal.agent).await;