[M6] Implement memory rollback recovery action

## Why
Paired with MemoryGuard vertical slice.

## Scope
- Roll back memory to last good checkpoint
- Evaluate recovery success

## Acceptance criteria
- [ ] Action implementation
- [ ] Integration with stale/conflict memory cases

## References
- Roadmap: `docs/roadmap.md`
- Related vertical slice: `experiments/results/tool-semantic-drift-001.md`
