mr store fix can report clean state while mr lock still fails with StoreHygieneError

We hit an inconsistent megarepo-store state while unwinding a downstream PR stack from a branch-pinned effect-utils ref back to canonical upstream main.

Observed behavior:

• downstream repos had temporarily pinned effect-utils to a PR branch ref for validation
• after the upstream PR merged, we switched the downstream config back to canonical upstream source
• mr store fix effect-utils reported no issues
• but mr lock and mr config ... still failed with StoreHygieneError
• the error still referenced the old effect-utils branch worktree and expected branch schickling/2026-03-21-effect-utils-upstream-nixpkgs-authority

So the commands disagreed:

• the repair command said the store was clean
• the normal lock/config commands still considered the store broken

Why this matters:
This is a normal workflow for megarepo PR stacks:

1. point downstream repos at an upstream PR branch/ref
2. validate downstream CI
3. merge upstream
4. switch downstream back to canonical upstream main
5. refresh lock files

Right now that last transition can get stuck in a state where mr store fix is a false negative and the only reliable recovery is manual lock editing.

Expected behavior:

• either mr store fix <member> should fully repair the stale store metadata so mr lock succeeds afterward
• or it should clearly report that unresolved hygiene problems remain

A minimal repro likely involves:

• a member pinned to a branch ref
• downstream config changed back to canonical upstream source
• store metadata still remembering the old branch/worktree binding

Related but not duplicate:

• #395
• #397

Acting on behalf of the user.

Reproduce the branch-ref to canonical-source transition, then run mr store fix effect-utils, mr lock, and the relevant mr config command to compare their hygiene checks. Inspect the store metadata that still references the old branch worktree; done means the fix either removes that stale binding so lock/config succeed or reports the remaining problem clearly.