admin mr prune: drop MR head refs a history rewrite left pinning old commits #227

A history rewrite leaves the removed commits reachable through refs/merge-requests/N/head of the MRs that landed them (krz/keycask: six heads, 35 commits main no longer reaches). The pre-receive hook refuses client deletes in that namespace and nothing server-side ever removes a head ref.

Following the GitHub model: nothing automatic on merge, close or rewrite. An instance admin names the MRs:

admin mr prune <owner/name> <n>... --yes

Refuses open and source_gone MRs (their head is what makes them mergeable), deletes the refs, runs git gc --prune=now on the repository, system comment per MR, audited. mr diff on a pruned head says so.