✨ feat(reviewer): use GitHub severity badges

[SigureMo]

What

• Replace Shields priority labels with GitHub's High / Medium / Low severity badges in dashboard findings and inline-review guidance.
• Standardize new findings on P1/P2/P3; P1 is the sole priority that can support REQUEST_CHANGES.
• Keep P0 only as a legacy dashboard-data alias, rendered as High.

Validation

• gh-slate render nyanpasu-review --config packages/nyanpasu-github-reviewer/src/nyanpasu_github_reviewer/templates/boards.toml --profile review --data packages/nyanpasu-github-reviewer/src/nyanpasu_github_reviewer/templates/review.example.json
• uv run pytest packages/nyanpasu-github-reviewer/tests (55 passed)
• git diff --check

^{This PR is co-authored with @copilot (gpt-6-luna xhigh)}

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, add credits to your account and enable them for code reviews in your settings.