A test pins reattaching a mirror: it refuses a different repository, or one already mirrored #7089

reattachMirror in src/actions/origins.ts refuses with not_the_same_repo and already_mirrored, and no test names any of them.

Done:

• [ ] A new test with the fake origin (test/fake-origin.ts): a mirror whose origin repository was renamed is reattached to it by its remote id, and the next sync works.
• [ ] Pointing it at another repository with a different remote id is not_the_same_repo. Pointing it at a remote another mirror already holds is already_mirrored.
• [ ] After each refusal, the mirror's remote and config are unchanged.

Files: test/mirror-reattach-refusals.test.ts

If the test finds the behaviour wrong, fix the action in the same pull and say so. If the check fails in a test this pull does not touch, name it in the pull rather than widening the change.

Before pushing: bun x tsc --noEmit and the new test.