Triage CLI: 'move back' affordance to revisit / undo previously applied actions

During a triage session the user should be able to **step back** and revisit a previously-applied (or previously-skipped) PR — to inspect what was applied, undo, or re-classify.

**Why:** mistakes happen; right now the only recovery is to manually re-find the PR on GitHub and reverse the change there.

### Open questions for design
- **Scope:** per-PR step-back only (one previous), per-session history (any earlier PR), or per-batch?
- **Undo semantics:** just review the action, or actively reverse the GitHub-side change (delete comment, re-add label, restore draft state)?
- **Storage:** in-memory session log only, or persisted across runs so a session interrupted and resumed still has its history?
- **UX:** dedicated key (`b` for back), or a menu entry on the post-action prompt?