coreapi: UPSTREAM #3 resolved upstream — record mirror remove rewire

entiredb refactored the mirror surface (ENT-741, on main; ENT-743 / entiredb#1830 merged): the by-mirror lookup is removed and delete is now DELETE /mirrors by upstream coords, dissolving the repoId-vs-mirrorId gap.

Our committed spec predates the refactor, so the generated client still exposes LookupRepoByMirror + DeleteMirror{MirrorID} and mirror remove still does the two-call lookup->delete. Rewrite #3 from a workaround note to a refresh-migration checklist: on the next spec refresh, mirror remove collapses to one delete-by-coords call and must surface the new status contract (404 no longer swallowed as idempotent success; 403/503/401).