Fix daemon auto-sync delete mutation not reflected in sync branch
When deleting an issue with `bd delete <id> --force`, the daemon auto-sync now properly removes the deleted issue from the sync branch.
**Problem:** The merge logic saw fewer local issues (due to deletion) and would re-add the deleted issue.
**Solution:** Add `ForceOverwrite` option to bypass merge logic when mutations occur. Mutation-triggered exports are authoritative and should overwrite, not merge.
Reviewed-by: stevey
eb988fcb21