fix: correct task scheduling and simplify agent internals

Fix queue cancellation, concurrency limits and worker status updates.

Use direct Explorer imports, split the Diagnoser loop into helpers and remove unused tool registration and prompt-builder code.

Correct model caching, endpoint selection and retry handling.

apps/admin_console/routers/tasks.py

@@ -228,6 +228,7 @@ async def stop_task(
         # Empty or non-JSON body: fall back to the query parameters.
         pass
 
+    # stop_tasks updates scheduler state and asyncio events owned by this loop.
     stopped = task_queue_service.stop_tasks(
         clear_all=target_all,
         session_id=target_sid,