fix(comments): keep label justifications current

[Takhoffman]

Summary

• record PR label syncs as ClawSweeper-owned churn so label-only updates do not look like target activity
• force durable review comment body refresh when ClawSweeper-owned label changes alter the rendered label explanations
• add regression coverage for PR label sync timestamps and recent stale comment refreshes

Root Cause

PR-specific label sync helpers returned only updated labels, so apply mode did not know those helpers had changed GitHub labels and did not write labels_synced_at. Separately, sync-comments-only could skip updating a recent durable comment because of comment_sync_min_age_days, leaving the latest visible comment without the newly-rendered label justifications even though labels had changed.

Validation

• pnpm run build
• pnpm exec node --test test/clawsweeper.test.ts
• pnpm exec oxfmt --check src/clawsweeper.ts test/clawsweeper.test.ts
• pnpm run check