Get an upload slot

Mint an upload slot for a script or sandbox that holds a file and cannot POST it to /media directly. Not part of the MCP surface since 2026-09-15: an assistant's attachment goes through create_post.files or media_pending and the approval card. Always returns {ticket, attach_url, max_bytes, expires_in_seconds}: attach_url takes the file straight from a BROWSER (POST multipart with a file field) and answers with the media_id directly. When a doorstep bucket is configured the reply ALSO carries {url, fields, curl}, a presigned path-style S3 POST: post the file there (the file field must come LAST), then redeem ticket at POST /media, and we collect the object, sniff its real type from the bytes and delete it. Nothing of yours is stored on S3. A slot lasts half an hour, covers several files, and a failed step does not use it up — fix the step and redeem the same ticket.