feat: add optional branch deletion after worktree remove

[guodong-sq]

Summary

• Add -b/--branch flag to wt remove that deletes the git branch after removing the worktree
• In interactive mode (no -y), prompt the user whether to delete the branch
• If safe delete (git branch -d) fails due to unmerged changes, offer force delete (git branch -D)
• -y without -b conservatively skips branch deletion entirely
• Branch checked out in main repo: warn and skip deletion
• Update help text, bash completions, and zsh completions
• Add 4 integration tests covering the new behavior

Test plan

• bats test/unit/ — all 151 tests pass
• bats test/integration/ — all 145 tests pass (12 existing + 4 new for wt-remove)
• wt remove -b deletes branch after worktree removal
• wt remove -y -b deletes branch silently
• wt remove -y does NOT delete branch (conservative)
• wt remove --merged still deletes branches (existing behavior preserved)

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