remove(ai_client): delete unused stub and proxy files

Deleted:
- src/ai_client_stub.py
- src/ai_client_proxy.py

Fixed test imports to use ai_client instead of ai_client_stub.

src/app_controller.py

@@ -18,7 +18,7 @@ from pathlib import Path
 from pydantic import BaseModel
 from typing import Any, List, Dict, Optional, Callable
 from src import aggregate
-from src import ai_client_stub as ai_client
+from src import ai_client
 from src import conductor_tech_lead
 from src import events
 from src import mcp_client
@@ -1625,7 +1625,7 @@ class AppController:
     Stops background threads and cleans up resources.
     [C: src/gui_2.py:App.run, src/gui_2.py:App.shutdown, tests/conftest.py:app_instance, tests/conftest.py:mock_app]
   """
-  from src import ai_client_stub as ai_client
+  from src import ai_client
   ai_client.cleanup()
   if hasattr(self, 'hook_server') and self.hook_server:
    self.hook_server.stop()
@@ -3009,7 +3009,7 @@ class AppController:
   self._update_cached_stats()
 
  def _update_cached_stats(self) -> None:
-  from src import ai_client_stub as ai_client
+  from src import ai_client
   self._cached_cache_stats = ai_client.get_gemini_cache_stats()
   self._cached_tool_stats = dict(self._tool_stats)
 
@@ -3017,7 +3017,7 @@ class AppController:
   """
     [C: src/gui_2.py:App._render_cache_panel]
   """
-  from src import ai_client_stub as ai_client
+  from src import ai_client
   ai_client.cleanup()
   self._update_cached_stats()