Refactor models and logging for nexus-claude-api

- Update default models in config to use `claude-opus-4.6`.
- Introduce logging configuration to write logs to a file.
- Add correlation ID to error responses for better traceability.
- Implement diagnostics for summarizing message requests.
- Normalize legacy system messages in the API.
- Enhance tests to cover new logging and error handling features.
- Update README and documentation to reflect changes in model defaults and logging behavior.

README.md

@@ -16,9 +16,12 @@ Use the printed Claude Code command in the same shell.
 
 `ANTHROPIC_AUTH_TOKEN='dummy'` in the printed command is only a local Claude Code compatibility placeholder. Claude Code expects an Anthropic auth token variable to exist, but this local proxy does not validate it by default. It is not your Nexus key.
 
+The printed command ends with `claude --model <configured-model>` so the current Claude Code process uses this proxy's configured model instead of any model saved in your global Claude settings. The helper does not edit `C:\Users\A200477427\.claude\settings.json` or any other global Claude Code configuration.
+
 ## Endpoints
 
 - `GET /`
+- `HEAD /`
 - `GET /health`
 - `GET /v1/models`
 - `POST /v1/messages`