A fork with an archived parent drops the 'following · N ahead' badge, and an admin can detach a fork from its parent #7452

A fork whose parent is archived still shows "following <parent> · N ahead", and it reads as unshipped work. trevato/forge shows "following trevato/forge-upstream · 1687 ahead". That parent is a frozen mirror from before the one-trunk switch (2026-10-15), archived today at the owner's request. The owner read the 1687 as "is anything shipping?".

Done when:

1. When the parent is archived, the repo page drops the following card and the ahead/behind badge, or shows one quiet line: "forked from trevato/forge-upstream (archived)". The forks list is the same.
2. A repo admin can "Detach from parent" in Settings. It repacks the fork so it owns its objects (the same detachFork the delete path uses), off the request path with progress, then clears parent_id. It's planned first, like archive.
3. Tests:

- An archived parent hides the badge. - Detach leaves the fork with no alternates link, still passing git fsck, and with no parent.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.