Add support for API key indexing in OpenAI compatibility clients
- Updated `NewOpenAICompatibilityClient` to accept `apiKeyIndex` for managing multiple API keys.
- Modified client instantiation loops to initialize one client per API key.
- Adjusted client ID format to include `apiKeyIndex` for unique identification.
- Removed API key rotation logic within `GetCurrentAPIKey`.
- Updated `.gitignore` to include `AGENTS.md`.
- Introduced `Watcher` for monitoring updates to the configuration file and authentication directory.
- Integrated file watching into `StartService` to handle dynamic changes without restarting.
- Enhanced API server and handlers to support client and configuration updates.
- Updated `.gitignore` to include `docs/` directory.
- Modified go dependencies to include `fsnotify` for the file watcher.