fix(bridge): forward Pi agent_end so the spinner stops at workflow completion

The FE derived 'working' purely as activeSession && !pendingWidget, so the
final workflow turn — the only one that ends without a follow-up gate —
left the spinner on forever (observed live: 21592s after F8 approve).

- SessionBridge maps Pi's agent_end -> SSE system_event {event: agent_end}
- PiProcessManager notifies the client (info error + synthetic agent_end)
  when the child dies unexpectedly; expected teardowns stay silent
- sessionStore tracks agentActive (on: user entry/text_delta/ui_request,
  off: agent_end); AppShell working now requires it; resume sets it
  optimistically

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/shell/AppShell.tsx

@@ -76,6 +76,8 @@ export function AppShell() {
     // working spinner shows straight away; the backend calls run after.
     setPanelSession(null);
     resetSession();
+    // Optimistic: the resume POST is about to hand the ball to the harness.
+    setAgentActive(true);
     setActiveSessionId(id);
     try {
       await resumeSession(id);
@@ -152,13 +154,17 @@ export function AppShell() {
     );
   }
 
-  // The harness "holds the ball" whenever a session is live and no widget is
-  // waiting on the human; once a widget appears, input is back in the user's court.
+  // The harness "holds the ball" whenever a session is live, no widget is waiting on
+  // the human, AND the Pi turn is still in flight (agentActive). Without the last
+  // condition the final workflow step — which ends with no follow-up gate — would
+  // leave the spinner on forever.
   const pendingWidget = useSessionStore((s) => s.pendingWidget);
   const resetSession = useSessionStore((s) => s.resetSession);
   const setPhase = useSessionStore((s) => s.setPhase);
+  const setAgentActive = useSessionStore((s) => s.setAgentActive);
   const lastSystemEvent = useSessionStore((s) => s.lastSystemEvent);
-  const working = Boolean(activeSessionId) && !pendingWidget;
+  const agentActive = useSessionStore((s) => s.agentActive);
+  const working = Boolean(activeSessionId) && !pendingWidget && agentActive;
   // Processing time counts only while the harness works, not while a finalized
   // session sits idle or a gate awaits the reviewer (pendingWidget).
   const running = working && !finalized;