Store the native session of a local mirror, so its owner's continuation resumes that exact conversation [write]

Only for a session created with local_mirror, and only by its owner. Body {file_id, synced_at?}: synced_at is the syncedAt receipt from PUT /mirror before capturing the archive. Without it, continuation uses the synced messages. The file is an asteroids.native-checkpoint v1 ZIP of at most 16 MiB (the files Claude Code or Codex keep for the conversation) that the owner uploaded to their personal files. It is checked whole (400 checkpoint_invalid), must be this chat's own conversation on its own agent (409 checkpoint_mismatch), and replaces the checkpoint stored before. A continuation the owner starts on the same agent resumes it natively while nothing was synced after it; otherwise it continues from the synced messages. A file the caller did not upload personally answers 404; a session that is not a mirror 409 not_a_mirror_session, one already continued 409 continued_in_cloud, an archived one 410. Nothing runs and nothing is billed.