Make one exported artifact public at an embeddable URL

Make one exported artifact PUBLIC at a clean, embeddable URL - the way to put a file in front of a human in another tool. Where export_file's presigned URL is private, unwieldy, and only good for curl, the share URL (https://s.givemeanode.io/shr-.../cat1.jpg) carries the real filename, the real content type, and no query string: paste it in Slack and an image unfurls inline; <img src> works; browsers render rather than download. Returns {share_id, url, filename, content_type, size_bytes, expires_at}. The bytes are FROZEN at share time (a copy, so the link outlives the export's ~30-day object), the link lives 365 days by default (expires_after: "48h".."365d" chooses; re-sharing the same artifact refreshes the same URL's expiry), and unshare(share_id) revokes it any time - list_shares shows everything your org has public. ANYONE holding the URL can fetch the file: sharing is a deliberate, audited decision, never a default. Images, PDFs, video, audio, and plain text render inline; HTML and SVG never do (they download instead); anything else is a clean named download. Directory exports share as a .tar. Bounded per org in size, rate, and count (refusals name the limit).