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.

tests/test_app_controller_offloading.py

@@ -88,7 +88,7 @@ def test_on_tool_log_offloading(app_controller, tmp_session_dir):
  script = "Get-Process"
  result = "Process list..."
  
- with patch("src.ai_client_stub.get_current_tier", return_value="Tier 3"):
+ with patch("src.ai_client.get_current_tier", return_value="Tier 3"):
   app_controller._on_tool_log(script, result)
   
   # Verify files were created in session directory