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.

docs/PRD.md

@@ -42,16 +42,14 @@ The first version will not include:
 
 The local API exposes:
 
-- `claude-sonnet-4.6`
 - `claude-opus-4.6`
-- `claude-haiku-4.5`
 
 Defaults:
 
-- Main model: `claude-sonnet-4.6`
-- Small model: `claude-haiku-4.5`
+- Main model: `claude-opus-4.6`
+- Small model: `claude-opus-4.6`
 
-`claude-sonnet-4.6` is the default because the AI Nexus documentation recommends it as the cost-effective default for most use cases.
+The proxy defaults to Opus because this deployment is intended for users whose Nexus access is limited to `claude-opus-4.6`. Sonnet and Haiku aliases are accepted for compatibility and resolved to Opus before calling Nexus.
 
 ## User Stories