Closes #2354
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<!-- This is an auto-generated description by cubic. -->
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## Summary by cubic
Automatically commits local changes before preparing a GitHub branch,
preventing failures on dirty repos and making branch setup reliable.
Closes #2354.

- **Bug Fixes**
  - Auto-add and commit pending changes in prepareLocalBranch.
  - Block with a clear error if a merge or rebase is in progress.
- Use locking and ensureCleanWorkspace to keep operations atomic and
clean.
  - Export prepareLocalBranch for reuse.

<sup>Written for commit b3af874e41856d47d397343ded7e278336ad0d92.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->