fix: preserve gitignored files during convert-to-worktree

[davidpoblador]

Summary

• Gitignored files (e.g. .env, secret/) were silently destroyed during convert-to-worktree because neither the copier nor manual path carries untracked files
• Now detects gitignored files before conversion, warns about them, and restores them to the project root after conversion
• Covers both copier and manual conversion paths, including cleanup on copier failure

Test plan

• Run tests/test_convert_preserves_gitignored.sh (validates root-level and nested gitignored files survive manual conversion)
• Manually test with copier path: create a repo with .env, run convert-to-worktree, verify .env is at project root
• Verify conversion without gitignored files still works (no regression)

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