feat(20-02): wire SSE subscription hooks into dashboard, detail, and inbox pages

Add useSubscription hooks to all three UI pages that invalidate React
Query caches on domain events:
- Dashboard: onTaskUpdate invalidates listInitiatives + listPhases
- Detail: onTaskUpdate invalidates phases/tasks/plans, onAgentUpdate
  invalidates listAgents
- Inbox: onAgentUpdate invalidates listWaitingAgents + listMessages

Subscription failures are silent (onError: () => {}) so pages degrade
gracefully to manual refresh when the backend is not running.

packages/web/src/routes/initiatives/$id.tsx

@@ -209,6 +209,23 @@ function InitiativeDetailPage() {
   const { id } = Route.useParams();
   const navigate = useNavigate();
 
+  // Live updates: invalidate detail queries on task/phase and agent events
+  const utils = trpc.useUtils();
+  trpc.onTaskUpdate.useSubscription(undefined, {
+    onData: () => {
+      void utils.listPhases.invalidate();
+      void utils.listTasks.invalidate();
+      void utils.listPlans.invalidate();
+    },
+    onError: () => {},
+  });
+  trpc.onAgentUpdate.useSubscription(undefined, {
+    onData: () => {
+      void utils.listAgents.invalidate();
+    },
+    onError: () => {},
+  });
+
   // State
   const [selectedTaskId, setSelectedTaskId] = useState<string | null>(null);
   const [taskCountsByPhase, setTaskCountsByPhase] = useState<