When adding a polecat, check if the branch already exists (e.g., from a
previous polecat that was removed but whose branch wasn't cleaned up).
If so, reuse the existing branch with WorktreeAddExisting instead of
failing with 'a branch named polecat/X already exists'.
Fixes: gt-bmjw
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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