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/server.py

@@ -19,6 +19,7 @@ Modular entrypoint for full trace inspection, step replay, and task execution ma
 
 import argparse
 import asyncio
+from contextlib import asynccontextmanager
 import os
 from pathlib import Path
 import secrets
@@ -95,8 +96,19 @@ except ImportError:
 # Initialize language server synchronization address
 init_ls_address()
 
+
+@asynccontextmanager
+async def _lifespan(_app: "FastAPI"):
+    """Start server resources and release them on shutdown."""
+    await on_startup()
+    try:
+        yield
+    finally:
+        await on_shutdown()
+
+
 # Initialize FastAPI application
-app = FastAPI(title="Artemis Admin & Trace Console")
+app = FastAPI(title="Artemis Admin & Trace Console", lifespan=_lifespan)
 LIFECYCLE_TOKEN = os.environ.get("ARTEMIS_LIFECYCLE_TOKEN") or secrets.token_urlsafe(32)
 app.state.lifecycle_token = LIFECYCLE_TOKEN
 
@@ -106,7 +118,6 @@ app.state.lifecycle_token = LIFECYCLE_TOKEN
 app.add_middleware(SameOriginBoundaryMiddleware)
 
 
-@app.on_event("startup")
 async def on_startup():
     """Startup lifecycle hooks."""
     state.is_shutting_down = False
@@ -150,7 +161,6 @@ async def on_startup():
     state.worker_task = asyncio.create_task(task_queue_service.queue_worker())
 
 
-@app.on_event("shutdown")
 async def on_shutdown():
     """Stop task and IPC children before the UI server exits."""
     state.is_shutting_down = True