Pack checks and first-claim handling

[Maximilian-Nesslauer]

Packs have no release host, so nothing mechanical can verify a first claim.

• an existing pack document is never modified or deleted; a new version is a new file
• the first account whose pull request creates packs/<id>/ owns the id, applied by a steward; later versions are verified against that account automatically by Privileged ownership and auto-merge workflow #4
• a broken pack version is retracted through index-status.toml, scoped to that version, and clients treat it like a yanked release