[checkpoint] Restore opencode_sandboxed_agent sessions #3284

Parent

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

Runtime state

responses_api_agents/opencode_sandboxed_agent/app.py owns OpenCode native session/database, sandbox filesystem, and process-local sandbox/result maps.

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

• Restore rebuilds both maps from durable state, resumes OpenCode, and reconciles invocation-owned calls from PR feat(observability): correlate OpenCode calls with invocations #2884.
• 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.