[checkpoint] Restore pinchbench sandbox runs

Parent

Child of #3222 and #3024. Depends on #3275.

Runtime state

responses_api_agents/pinchbench/app.py owns sandbox filesystem, transcript/session tree, run/work directories, and result cursor.

Adapter requirements

Persist the logical execution-to-runtime mapping, provider/version and physical generation, sandbox snapshot/reference, image/spec and mount fingerprints, agent or verifier cursor, and result-manifest identity. Rebind a replacement runtime atomically and fence the old handle. Never claim that a filesystem-only copy restores unsupported live process state.

Acceptance criteria

• Transcript and workspace restore under stable identity and the final result is neither lost nor emitted twice.
• Mutate runtime state, commit, replace both application and runtime process where supported, restore, and compare the next operation/result.
• Stale handles and delayed old-runtime results are rejected.
• Partial restore activates no execution mapping.
• Duplicate retirement and failed-provider cleanup release the restored runtime exactly once.

Read responses_api_agents/pinchbench/app.py first, then review the parent issues #3222 and #3024 and dependency #3275 for the runtime boundaries. The work is done when transcript and workspace identity, mappings, cursors, and result manifests survive supported replacement, while stale or partial restores are rejected and cleanup occurs exactly once.