fix: re-detect sessions after idle cleanup

Added forgetSession() to SessionMonitor. When watcher marks a session
idle/done, it now clears the key from the monitor's known sessions map.
Next poll cycle re-detects the session if the transcript is still active,
creating a fresh status post.

src/session-monitor.js

@@ -67,6 +67,15 @@ class SessionMonitor extends EventEmitter {
     if (this.logger) this.logger.info('SessionMonitor stopped');
   }
 
+  /**
+   * Remove a session from known sessions so it can be re-detected on next poll.
+   * Called when the watcher marks a session as idle/done.
+   * @param {string} sessionKey
+   */
+  forgetSession(sessionKey) {
+    this._knownSessions.delete(sessionKey);
+  }
+
   /**
    * Get all agent directories under transcriptDir.
    * @private