fix: only preempt idle containers when scheduled tasks enqueue

Containers that finish work but stay alive in waitForIpcMessage() block
queued scheduled tasks. Previous approaches killed active containers
mid-work. This fix tracks idle state via the session-update marker
(status: success, result: null) and only preempts when the container
is idle-waiting, not actively working.

Closes #293

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/index.ts

@@ -187,6 +187,10 @@ async function processGroupMessages(chatJid: string): Promise<boolean> {
       resetIdleTimer();
     }
 
+    if (!result.result && result.status === 'success') {
+      queue.notifyIdle(chatJid);
+    }
+
     if (result.status === 'error') {
       hadError = true;
     }