agents get a kickoff inbox message on start/restart/rebuild

new Coordinator::kick_agent(name, reason) drops a system message
into the agent's inbox so the next turn picks it up with a 'you
were just (re)started, check /state/ for notes, --continue session
is intact' hint. wakes the turn loop without any harness-side
handling needed — it's just another inbox message with sender =
'system'.

wired from:
- dashboard /start /restart /rebuild handlers (via lifecycle_action's
  on-success tail)
- manager mcp_hyperhive_start / restart

dashboard: pending approvals + tombstones + questions now refresh on
a 5s heartbeat when nothing else is happening. previously refresh
only fired on async-form submit or on broker traffic addressed to
operator — manager-queued approvals went through neither, so the
operator had to reload to see them. 5s is the slow-path; 2s
remains for in-flight transients.

hive-c0re/src/manager_server.rs

@@ -162,7 +162,10 @@ async fn dispatch(req: &ManagerRequest, coord: &Coordinator) -> ManagerResponse
                 };
             }
             match lifecycle::start(name).await {
-                Ok(()) => ManagerResponse::Ok,
+                Ok(()) => {
+                    coord.kick_agent(name, "container started");
+                    ManagerResponse::Ok
+                }
                 Err(e) => ManagerResponse::Err {
                     message: format!("{e:#}"),
                 },
@@ -176,7 +179,10 @@ async fn dispatch(req: &ManagerRequest, coord: &Coordinator) -> ManagerResponse
                 };
             }
             match lifecycle::restart(name).await {
-                Ok(()) => ManagerResponse::Ok,
+                Ok(()) => {
+                    coord.kick_agent(name, "container restarted");
+                    ManagerResponse::Ok
+                }
                 Err(e) => ManagerResponse::Err {
                     message: format!("{e:#}"),
                 },