fix: make _get_loop_agent singleton thread-safe with double-checked locking

Fixes #446

src/timmy/agentic_loop.py

@@ -18,6 +18,7 @@ from __future__ import annotations
 import asyncio
 import logging
 import re
+import threading
 import time
 import uuid
 from collections.abc import Callable
@@ -59,6 +60,7 @@ class AgenticResult:
 # ---------------------------------------------------------------------------
 
 _loop_agent = None
+_loop_agent_lock = threading.Lock()
 
 
 def _get_loop_agent():
@@ -69,9 +71,11 @@ def _get_loop_agent():
     """
     global _loop_agent
     if _loop_agent is None:
-        from timmy.agent import create_timmy
+        with _loop_agent_lock:
+            if _loop_agent is None:
+                from timmy.agent import create_timmy
 
-        _loop_agent = create_timmy()
+                _loop_agent = create_timmy()
     return _loop_agent