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/task-scheduler.ts

@@ -121,6 +121,9 @@ async function runTask(
           // Only reset idle timer on actual results, not session-update markers
           resetIdleTimer();
         }
+        if (!streamedOutput.result && streamedOutput.status === 'success') {
+          deps.queue.notifyIdle(task.chat_jid);
+        }
         if (streamedOutput.status === 'error') {
           error = streamedOutput.error || 'Unknown error';
         }