feat(HAI-003): add file-system watcher to TaskStore for live dashboard updates

- Add watch() method using Node's built-in fs.watch with recursive option
- Detect task.json changes and emit appropriate SSE events (created/moved/updated/deleted)
- Debounce per-file changes (150ms) to coalesce rapid writes
- Suppress duplicate events for in-process mutations via recentlyWritten set
- Integrate watcher into dashboard server startup with clean SIGINT shutdown
- Add @types/node to core package for proper TypeScript support

pnpm-lock.yaml

@@ -36,6 +36,9 @@ importers:
 
   packages/core:
     devDependencies:
+      '@types/node':
+        specifier: ^25.5.0
+        version: 25.5.0
       typescript:
         specifier: ^5.7.0
         version: 5.9.3