Implement durable handoff snapshots between sessions

[xodapi]

Why

The agent should be able to resume from repo-local durable state instead of reconstructing everything from chat. This matters after interrupted sessions, restarts, or when a different agent continues work.

Proposed behavior

Add a command or runtime hook that writes a privacy-safe handoff snapshot. The exact backing store should follow the decision in #1, with a likely staged path: inspectable JSONL/latest-state first, then redb if the Rust helper becomes the state boundary.

Snapshot should include:

• current repo and branch;
• fork/upstream remotes and current PR URL if known;
• linked GitHub issue numbers relevant to the current work;
• last npm test, secret-scan, runtime-smoke, and proxy-status result summaries;
• current proxy status from /metrics or proxy-status;
• explicit next action and known blockers.

Privacy boundary

Do not store prompts, model responses, API keys, local chat text, secret file contents, or raw private logs.

Acceptance criteria

• A snapshot can be written and read without the proxy running.
• Snapshot output is available as human text and JSON.
• Tests assert that prompts, responses, and keys are not stored.
• README documents how to use it after an interrupted session.
• The implementation references the storage decision from Design durable session and project state store #1.