fix(list_worktrees): detect detached via 'detached' marker, not HEAD line

[RodrigoEspinosa]

🎯 What: The testing gap addressed
The list_worktrees function in bin/wt lacked test coverage, which could lead to regressions.

📊 Coverage: What scenarios are now tested
The new tests/list_worktrees.bats file covers four main scenarios:

1. Listing all worktrees when no target is specified.
2. Fetching the path for a specific branch.
3. Fetching the path for a detached head using the (detached) specifier.
4. Correctly outputting nothing for a non-existent branch.

✨ Result: The improvement in test coverage
The codebase now ensures list_worktrees accurately maps branch names, generic strings, and paths correctly under various git worktree outputs, including detached states. It also allowed fixing an implicit bug where list_worktrees misidentified any worktree as detached because git worktree list --porcelain prints a HEAD block for all worktrees; now it explicitly looks for the detached keyword.

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PR created automatically by Jules for task 12611007312094146683 started by @RodrigoEspinosa

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