[ecr] use Claude Opus 5.5 for the coordinator, Sonnet 5.5 as the step down

[LipLoody]

Why

The coordinator still ran on Haiku 4.5 while every specialist pass had moved to Opus 5.5, and the docs still pointed at the legacy Sonnet 5.

How

Use anthropic/claude-opus-5-5 for the repo coordinator and name anthropic/claude-sonnet-5-5 as the cheaper model in the README, the setup reference, and the model id error message.

Test Plan

After merge, run a review and check that the Models used line reports anthropic/claude-opus-5-5 for coordinator. Then set REVIEWER_MODEL=anthropic/claude-sonnet-5-5 on one run and check the review finishes and reports that model.

[github-actions]

🤖 AI code review

Decision: Ready for human review (with comments)

Overall PR risk: Low. This change modifies the coordinator model selection and documentation, aligning the repo coordinator with Opus 5.5 and updating fallback model guidance. The affected surface includes README.md, error messages, and skill references. The primary issue is that the recommended fallback model id anthropic/claude-sonnet-5-5 does not exist; users following the documentation will fail preflight validation.

🔗 Review setup:

• This PR changes code the reviewer prompts cite (src, src/config/load.ts). Check that the guidance quoting it is still correct.

🟡 Warning (1)

• Recommended model id anthropic/claude-sonnet-5-5 does not exist in known Claude catalog — README.md:520 (correctness) · id:47c65550046a
  Confidence: Medium — the known Claude model list names Sonnet 5 as claude-sonnet-5 and has no Sonnet 5.5 id. The repo tests use only claude-sonnet-5 as the valid Sonnet id.
  Impact if shipped: Medium — users who follow the README or setup skill can configure this id, which preflight rejects, blocking the review.
  
  

  Details

  Evidence and reasoning

  The PR changes the recommended cheaper model from anthropic/claude-sonnet-5 to anthropic/claude-sonnet-5-5 in README.md, the setup skill reference, and the formatUnknownModels error message. The known Claude model ids are claude-opus-5-5, claude-sonnet-5, and claude-haiku-4-5-20251001. There is no claude-sonnet-5-5.

  When a user follows the README and sets model or REVIEWER_MODEL to anthropic/claude-sonnet-5-5, the preflight check flags it as unknown, and the review fails. The error message then shows this same invalid id as the example of a correct provider/model id, directing the user back to the broken configuration.

  Suggestion: Change the id back to anthropic/claude-sonnet-5 in README.md, ecr-reference.md, src/core/opencode.ts, and the src/config/load.ts comment. Alternatively, confirm with opencode models that the provider serves claude-sonnet-5-5 before documenting it.

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This review is advisory — it never blocks a merge and never auto-approves.