fix: ghost-watch reactivation for consistent session restart

Problem: after a session completes, removeSession() deleted the file→session
mapping. When the next user message caused the JSONL to be written, fs.watch
fired but fileToSession returned undefined — silently dropped. Reactivation
only happened on the next session-monitor poll (up to 2s later), and by then
the watcher had missed the first lines of the new turn.

Fix:
- removeSession() keeps the file in fileToSession as a ghost marker
- fs.watch fires → ghost detected → emit 'session-reactivate'
- watcher-manager clears completedSessions cooldown + calls pollNow()
- session-monitor re-detects immediately with no poll lag
- Ghost removed after first fire (one-shot)

Also adds SessionMonitor.pollNow() for forced immediate polling.

src/session-monitor.js

@@ -108,6 +108,14 @@ class SessionMonitor extends EventEmitter {
     this._completedSessions.delete(sessionKey);
   }
 
+  /**
+   * Trigger an immediate poll without waiting for the next interval tick.
+   * Used by ghost-watch reactivation to avoid up to pollMs latency.
+   */
+  pollNow() {
+    this._poll();
+  }
+
   /**
    * Get all agent directories under transcriptDir.
    * @private