[BUG] Org plugin marketplaces only accept private repos, public repos are rejected

Adding a plugin marketplace to an organization account only works with a private repo. Public repos are rejected server-side:

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "Only private and internal repositories can be used for marketplaces.",
    "details": { "error_visibility": "user_facing" }
  },
  "request_id": "req_011CdTQTh1j8iVJzv8jwnDDc"
}

The UI just shows Failed to create marketplace, so the actual reason is only visible in the network tab even though the API marks it user_facing.

This is poor UX. Public repos are the normal way to distribute a marketplace, the CLI (/plugin marketplace add owner/repo) accepts them, and the only workaround is mirroring a public repo into a private one and keeping it in sync by hand.

Ask: allow public repos as org marketplace sources. If that restriction is intentional, at least surface the real message in the UI.

Surface: claude.ai web / Cowork, organization account.