Add persistent workflow recovery and hosted samples

[shibayan]

Priority

P1

Goal

Persist workflow checkpoints and demonstrate recovery, handoff, and multi-step hosted scenarios.

Scope

• Implement a production-oriented checkpoint store adapter and a local development adapter.
• Restore workflow execution after process restart.
• Add hosted samples for simple workflow and handoff scenarios.
• Document checkpoint cleanup, retention, and incompatible-version handling.

Acceptance criteria

• An interrupted workflow resumes from its last committed checkpoint.
• Checkpoints remain isolated by user, tenant, and execution.
• Duplicate delivery does not repeat committed workflow effects.
• Simple and handoff samples run end-to-end.
• Recovery behavior is covered by restart integration tests.

Dependencies

Blocked by the workflow contracts and Hosted Responses workflow integration.