feat: lock file deletion = instant done signal

When the gateway deletes .jsonl.lock it means the final reply was sent.
Use this as an immediate 'turn complete' trigger instead of waiting for
cache-ttl (3s grace) or idle timeout (60s).

- status-watcher.js: _onFileChange checks if .lock file exists on event.
  If deleted -> emits 'session-lock-released'. Added triggerIdle() which
  cancels all timers and emits session-idle immediately.
- watcher-manager.js: wires session-lock-released/session-lock-released-path
  to watcher.triggerIdle() for instant completion.

Combined with lock-created trigger (previous commit), the full lifecycle is:
  User sends message
    -> .jsonl.lock created -> status box appears immediately
    -> JSONL writes -> status box updates in real time
    -> Gateway sends reply -> .jsonl.lock deleted -> status box marks done instantly

src/status-watcher.js

@@ -225,19 +225,31 @@ class StatusWatcher extends EventEmitter {
     // Emit 'session-lock' so watcher-manager can trigger immediate reactivation
     // without waiting for the first JSONL write.
     if (fullPath.endsWith('.jsonl.lock')) {
-      // Derive the JSONL path and find the session key
       const jsonlPath = fullPath.slice(0, -5); // strip '.lock'
-      const sessionKey = this.fileToSession.get(jsonlPath) ||
-        this.fileToSession.get('\x00ghost:' + jsonlPath.replace('\x00ghost:', ''));
-      if (sessionKey) {
-        const originalKey = sessionKey.startsWith('\x00ghost:') ? sessionKey.slice(7) : sessionKey;
-        if (this.logger) {
-          this.logger.info({ sessionKey: originalKey }, 'Lock file detected — session started, triggering early reactivation');
+      const lockExists = (() => { try { fs.statSync(fullPath); return true; } catch(_) { return false; } })();
+
+      // Resolve session key from known sessions or ghost watch
+      let sessionKey = this.fileToSession.get(jsonlPath);
+      if (sessionKey && sessionKey.startsWith('\x00ghost:')) sessionKey = sessionKey.slice(7);
+
+      if (lockExists) {
+        // Lock file CREATED — gateway started processing user message.
+        // Earliest possible signal: fires before any JSONL write.
+        if (sessionKey) {
+          if (this.logger) this.logger.info({ sessionKey }, 'Lock file created — session active, triggering early reactivation');
+          this.emit('session-lock', sessionKey);
+        } else {
+          this.emit('session-lock-path', jsonlPath);
         }
-        this.emit('session-lock', originalKey);
       } else {
-        // Unknown session — emit with path so watcher-manager can look it up
-        this.emit('session-lock-path', jsonlPath);
+        // Lock file DELETED — gateway finished the turn and sent final reply.
+        // Immediate idle signal: no need to wait for cache-ttl or 60s timeout.
+        if (sessionKey) {
+          if (this.logger) this.logger.info({ sessionKey }, 'Lock file deleted — turn complete, marking session done immediately');
+          this.emit('session-lock-released', sessionKey);
+        } else {
+          this.emit('session-lock-released-path', jsonlPath);
+        }
       }
       return;
     }
@@ -490,6 +502,22 @@ class StatusWatcher extends EventEmitter {
    * Schedule an idle check for a session.
    * @private
    */
+  /**
+   * Immediately mark a session as done without waiting for idle timeout.
+   * Called when the lock file is deleted — the gateway has sent the final reply.
+   * @param {string} sessionKey
+   */
+  triggerIdle(sessionKey) {
+    const state = this.sessions.get(sessionKey);
+    if (!state) return;
+    // Cancel any pending timers
+    if (state.idleTimer) { clearTimeout(state.idleTimer); state.idleTimer = null; }
+    if (state._filePollTimer) { clearInterval(state._filePollTimer); state._filePollTimer = null; }
+    if (this.logger) this.logger.info({ sessionKey }, 'triggerIdle: lock released — marking session done immediately');
+    state.status = 'done';
+    this.emit('session-idle', sessionKey, this._sanitizeState(state));
+  }
+
   _scheduleIdleCheck(sessionKey, state) {
     if (state.idleTimer) {
       clearTimeout(state.idleTimer);

src/watcher-manager.js

@@ -387,7 +387,6 @@ async function startDaemon() {
 
   // Lock file for a session the watcher doesn't know about yet — look it up by path
   watcher.on('session-lock-path', (jsonlPath) => {
-    // Find the session key by matching sessionFile in sessions.json
     const sessionKey = monitor.findSessionByFile(jsonlPath);
     if (sessionKey) {
       logger.info({ sessionKey }, 'Lock file (path lookup): session activated — early reactivation');
@@ -396,6 +395,22 @@ async function startDaemon() {
     }
   });
 
+  // ---- Lock file released: turn complete, mark done immediately ----
+  // Gateway deletes .jsonl.lock when the final reply is sent to the user.
+  // This is the most reliable "done" signal — no waiting for cache-ttl or 60s idle.
+  watcher.on('session-lock-released', (sessionKey) => {
+    logger.info({ sessionKey }, 'Lock file released — turn complete, triggering immediate idle');
+    watcher.triggerIdle(sessionKey);
+  });
+
+  watcher.on('session-lock-released-path', (jsonlPath) => {
+    const sessionKey = monitor.findSessionByFile(jsonlPath);
+    if (sessionKey) {
+      logger.info({ sessionKey }, 'Lock file released (path lookup) — triggering immediate idle');
+      watcher.triggerIdle(sessionKey);
+    }
+  });
+
   // ---- Session Update (from watcher) ----
   watcher.on('session-update', (sessionKey, state) => {
     const box = activeBoxes.get(sessionKey);