feat(ai_client): isolation of current_tier using threading.local() for parallel agent safety

src/app_controller.py

@@ -851,7 +851,7 @@ class AppController:
       self._api_event_queue.append({"type": "response", "payload": payload})
  def _handle_request_event(self, event: events.UserRequestEvent) -> None:
   """Processes a UserRequestEvent by calling the AI client."""
-  ai_client.current_tier = None # Ensure main discussion is untagged
+  ai_client.set_current_tier(None) # Ensure main discussion is untagged
   if self.ui_auto_add_history:
    with self._pending_history_adds_lock:
     self._pending_history_adds.append({
@@ -935,7 +935,7 @@ class AppController:
 
  def _on_tool_log(self, script: str, result: str) -> None:
   session_logger.log_tool_call(script, result, None)
-  source_tier = ai_client.current_tier
+  source_tier = ai_client.get_current_tier()
   with self._pending_tool_calls_lock:
    self._pending_tool_calls.append({"script": script, "result": result, "ts": time.time(), "source_tier": source_tier})