Introduce a lightweight checkpoint/rollback system for sandbox state
during plan execute and apply flows. CheckpointManager snapshots
the sandbox working directory before each phase and can restore it
on failure, giving the execution engine a reliable undo mechanism.
Key changes:
- SandboxCheckpoint model, Checkpointable protocol, and
CheckpointManager in infrastructure/sandbox/checkpoint.py
- PlanExecutor gains optional checkpoint_manager with pre/post
execute hooks and automatic rollback on failure
- PlanApplyService gains optional checkpoint_manager with pre-apply
checkpoint and rollback helper
- 12 BDD scenarios (features/sandbox_checkpoints.feature)
- 5 Robot Framework smoke tests (robot/sandbox_checkpoint_smoke.robot)
- ASV benchmarks for creation, rollback, and listing operations
- Reference documentation in docs/reference/sandbox.md
ISSUES CLOSED: #183
