Add coverage for the uninstall branch where `local` has already been moved to
`.local.backup`, but deleting that backup fails. The Unix-gated test makes a
child directory non-writable before uninstall starts, so recursive deletion of
the renamed backup fails without adding production hooks.
The test verifies rollback restores the previous local install, removes the
backup path, and clears the intent. It is gated to Unix because deletion
permission behavior is platform-specific; Windows coverage would need a
different failure mechanism rather than pretending this setup is portable.
Test Plan:
- git diff --check
- just fmt
- just clippy
- just test
Follow-up-Plan: FOLLOW_UP_2.md
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