refactor: remove all AI-specific protobuf definitions and dead code

- Remove AI-specific gRPC methods from loader.proto (GenerateSQL, ValidateSQL, GetAICapabilities)
- Remove AI-specific structures from server.proto (AICapabilitiesResponse, GenerateSQLRequest, etc.)
- Remove AI-specific test files (ai_proto_test.go, ai_loader_test.go)
- Remove AI performance testing script (ai_performance_test.sh)
- Regenerate all protobuf files with cleaned definitions
- Eliminate 4000+ lines of dead code that violated unified plugin architecture

This completes the migration to unified plugin management where AI plugins
follow the same patterns as all other plugins through ExtManager interface
and Categories field classification.

Author: KariHall619