Validate denied and stale PR confirmations

[primorLee]

Summary

• prove denied and stale confirmation attempts emit no authorization file
• verify merge, auto-merge, force-push, mark-ready, and default-branch writes remain unauthorized
• document the fresh-task result and local-only handoff for PRM-009

Verification

• Python 3.11 and 3.12: 74 tests passed, 2 sandbox socket tests skipped, 80% branch coverage
• Ruff, formatting, mypy, compileall, Skill validation, and Plugin validation passed
• an installed-Plugin fresh task stopped on explicit denial and a stale packet

Assessment and safety

PRman result: abstain because no production scorer or trusted evidence attestation is configured. This Draft PR acknowledges that quality readiness remains unknown. It does not authorize merge, auto-merge, mark-ready, force-push, or default-branch writes. CI monitoring is read-only; at most two directly related CI repair updates are within the confirmed envelope.