[purp]

Summary

Review Readiness failures now report their cause as a GitHub error annotation, so the run page and PR checks list show why the check failed instead of only "Process completed with exit code 1."

Related Issue

Closes #5573

The linked issue lacks state:accepted; the author has ADMIN access to this repository.

Changes

• check_review_readiness.py prints ::error title=Review Readiness: <category>::<message> when GITHUB_ACTIONS is true. Outside Actions the output is the same plain Review readiness: … line as before.
• gh api failures name the step that failed (for example, "Requesting reviewers failed with a GitHub API error"), then give the method, path, exit status and the error text gh printed. The --input payload is never printed.
• Label failures say what is wrong with this PR's labels, name them, and say how to fix them. Once the labeling agent can repair these, feat(review): let the labeling agent repair label defects before Review Readiness fails #5578 removes that guidance.
• Which cases fail the check is unchanged; failures still exit 1.

Release Notes

Testing

• Checks appropriate to the affected code and behavior pass
• Unit tests added/updated (if applicable)
• E2E tests added/updated (if applicable)

mise run test:review-readiness passes (58 tests, with and without GITHUB_ACTIONS=true), ruff check and ruff format --check pass, and the pre-commit hook passed on each commit. The workflow runs on pull_request_target and uses the script from main, so this PR's own run does not exercise the change. The annotation on a real run can be confirmed after merge by giving a ready PR two area: labels.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)