repo mirror remove: record the repoId==mirrorId verification

Follow-up on the by-mirror-repoId-to-DeleteMirror ambiguity. Confirmed live that the by-mirror lookup's repoId is byte-identical to the same repo's mirrorId from list, so DeleteMirror(repoId) is correct. Expanded the comment to note (a) the generated API offers no delete-by-coords route, so the lookup is the only id source — the invariant is unavoidable, not a choice; (b) it's verified live; and (c) the client-contract ambiguity dissolves upstream if the lookup response names the field mirrorId or a delete-by-coords route is added.

A unit test can't assert a server invariant and a network test doesn't belong in the offline suite, so the durable backing is the verification note plus the named upstream fix.