Fix detection of dirty working directory

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Summary

• ensure dirty_working_directory also detects untracked files
• document logic for using git status
• add tests verifying backup, merge and rebase fail with untracked files present

Testing

• cargo fmt --all
• cargo test --quiet -- --test-threads=1 | tail -n 20

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https://chatgpt.com/codex/tasks/task_e_6827bf5735e48330b24993762023a769

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Closing as stale: targets pre-refactor src/main.rs (the function now lives at src/git_chain/core.rs) and no longer merges. The core change — treating untracked files as a dirty working directory — would make git-chain stricter than git itself: git rebase/merge/checkout all proceed with untracked files present and refuse only on actual collisions, which git-chain already inherits from its git subprocesses and safe checkouts. Blocking every chain operation on any untracked scratch file would be a UX regression with no safety gain. The error-message improvement from this PR (naming the current branch) is being applied separately.