fix: delete+recreate status post, file polling fallback

- StatusBox: delete+recreate instead of PUT to keep post at thread bottom
  (Mattermost clears pin on PUT and doesn't bump edited posts)
- StatusBox: extends EventEmitter, emits 'post-replaced' events
- StatusWatcher: 500ms file polling fallback (fs.watch unreliable on
  Docker bind mounts / overlay fs)
- WatcherManager: handles post-replaced events to update activeBoxes
- SessionMonitor: forgetSession() for idle session re-detection

src/watcher-manager.js

@@ -282,6 +282,18 @@ async function startDaemon() {
     logger.debug({ sessionKey }, 'Session removed from sessions.json');
   });
 
+  // ---- Post Replaced (delete+recreate) ----
+  sharedStatusBox.on('post-replaced', (oldPostId, newPostId) => {
+    // Update activeBoxes to point to new post ID
+    for (var entry of activeBoxes.entries()) {
+      if (entry[1].postId === oldPostId) {
+        entry[1].postId = newPostId;
+        logger.debug({ sessionKey: entry[0], oldPostId, newPostId }, 'Post replaced (delete+recreate)');
+        break;
+      }
+    }
+  });
+
   // ---- Session Update (from watcher) ----
   watcher.on('session-update', (sessionKey, state) => {
     const box = activeBoxes.get(sessionKey);