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
 

docs/REQUIREMENTS_DESIGN.md

@@ -60,8 +60,8 @@ Options:
 - `--port`, `-p`: default `4141`
 - `--endpoint-url`: default `https://genai-nexus.api.corpinter.net`
 - `--api-key`: optional; fallback to ignored local config, `NEXUS_API_KEY`, then `AWS_BEARER_TOKEN_BEDROCK`
-- `--model`: default `claude-sonnet-4.6`
-- `--small-model`: default `claude-haiku-4.5`
+- `--model`: default `claude-opus-4.6`
+- `--small-model`: default `claude-opus-4.6`
 - `--claude-code`: print Claude Code launch command
 - `--verbose`, `-v`: debug logging without secrets
 
@@ -95,15 +95,15 @@ Inbound authentication headers are accepted for compatibility but not validated
 
 Public local model IDs:
 
-- `claude-sonnet-4.6`
 - `claude-opus-4.6`
-- `claude-haiku-4.5`
 
-Backend IDs are resolved through a mapping table. The initial default mapping keeps the same IDs, except common short aliases are supported:
+Backend IDs are resolved through a mapping table. This deployment exposes Opus and maps common Sonnet/Haiku aliases to Opus for users whose Nexus access is limited to `claude-opus-4.6`:
 
-- `claude-sonnet-4` -> `claude-sonnet-4.6`
+- `claude-sonnet-4.6` -> `claude-opus-4.6`
+- `claude-haiku-4.5` -> `claude-opus-4.6`
+- `claude-sonnet-4` -> `claude-opus-4.6`
 - `claude-opus-4` -> `claude-opus-4.6`
-- `claude-haiku-4` -> `claude-haiku-4.5`
+- `claude-haiku-4` -> `claude-opus-4.6`
 
 If Nexus requires different backend IDs, update the mapping without changing Claude Code-facing model IDs.