Document PR fast-track triage criteria

[Copilot]

Contributor guidance did not explain the existing PR triage labels or how risk and priority influence review routing. This makes fast-track eligibility and alternatives hard to discover.

• PR triage labels
  • Documents pr-risk:*, pr-priority:*, and pr-action:* labels.
• Review routing
  • Defines criteria for fast_track, batch_review, and defer.
  • Clarifies that fast-track expedites human review without bypassing CI or approvals.

- `pr-action:fast_track` identifies a high-value, high-priority PR
  ready for expedited human review.
- `pr-action:batch_review` groups similar low- or medium-risk PRs.
- `pr-action:defer` identifies lower-value, lower-priority work.

• Fixes [deep-report] Document fast-track PR review criteria in contributor guide #53873

[github-actions]

Great work! 🚀 This PR documents an important but previously undiscovered part of the PR review process. The guidance clearly explains:

• How pr-risk:*, pr-priority:*, and pr-action:* labels work together for triage
• What fast_track, batch_review, and defer labels mean in practice
• The crucial clarification that fast-track expedites review without bypassing CI or required approvals

This directly addresses the gap identified in issue #53873 and makes the existing (implicit) triage system discoverable for both human and AI contributors. The addition is focused, well-scoped (15 lines), and positioned correctly in the CONTRIBUTING.md guide.

✅ Ready for review — no additional work needed.

Generated by ✅ Contribution Check · auto · 59.7 AIC · ⌖ 4.1 AIC · ⊞ 9.2K · ◷

[github-actions]

Triage: category=docs · risk=low · score=32/100 (impact 10/50, urgency 8/30, quality 14/20)
Recommended action: batch_review

Docs-only change to CONTRIBUTING.md (+15/-0). Draft — safe to undraft and merge quickly; batch with other doc/small fixes.

Generated by 🔧 PR Triage Agent · auto · 58.8 AIC · ⌖ 2.56 AIC · ⊞ 8.3K · ◷