refactor: remove AI plugin implementation files from main project

These files should be implemented in separate AI plugin repositories:
- ai_plugin_bridge.go: AI plugin bridge implementation
- ai_query_router.go: AI query routing logic
- ai_server_test.go: AI server tests
- ai_bridge_simple_test.go: AI bridge tests

Following the decoupled architecture where main project only provides
interfaces and management capabilities, while AI plugins contain the
actual implementation logic in separate codebases.

Author: KariHall619