feat(FN-2274): add engine-level stale-task safeguards in heartbeat execution

- HeartbeatMonitor.executeHeartbeat() now checks if resolved task is done/archived
  and exits before session creation with reason 'task_closed'
- When stale task came from persisted agent.taskId, clears the linkage to prevent
  repeated stale activations
- HeartbeatTriggerScheduler.watchAssignments() now skips assignment callback dispatch
  when assigned task is done/archived (when taskStore is available)
- Added comprehensive tests for stale-task activation guard behavior

packages/dashboard/vite.config.ts

@@ -50,7 +50,11 @@ export default defineConfig({
   },
   server: {
     proxy: {
-      "/api": "http://localhost:3000",
+      "/api": {
+        target: `http://localhost:${process.env.FUSION_API_PORT ?? "4040"}`,
+        changeOrigin: true,
+        ws: true,
+      },
     },
   },
 });