An earlier export's record and a fresh download URL

Fetch an artifact's record and a fresh download URL by id - the way back to an export after the original export_file response is gone, and the poll for an export that export_file answered with state "pending" (list_artifacts and a mission page list artifact ids; this turns one back into bytes). Returns the export_file shape: {artifact_id, state, source_path, size_bytes, sha256, download: {url, expires_at}} for a ready artifact (a fresh ~7-day presigned GET; the stored object itself lives ~30 days, after which the download is omitted and re-exporting is the fix). A pending artifact answers state "pending" and next_step (this is not an error: call again in a few seconds). A failed one answers its error plus error_code and retryable (the export_file refusal vocabulary). A capture that landed in your own store or platform storage shows where it went instead of a link. To put the file in front of a human (Slack, a doc, <img src>), share_artifact mints a public embeddable link instead.