恢复被删的 dsh 会话

undohaven restore --to <snapshot-id> --scope sessions。会话在 sessions/**/session.vN.jsonl.zstd。只恢复 sessions 范围时，其他配置不动。

undohaven restore --to <snapshot-id> --scope sessions. Session logs live under sessions/**/session.vN.jsonl.zstd. Restoring the sessions scope writes those files back and leaves unrelated config alone.