fix: skip empty .git directories during repo discovery

[kayagokalp]

Summary

• Validate .git directories contain a HEAD file before treating them as repo roots
• Empty .git dirs (e.g. from docker-compose volume mounts) are now skipped during tree-walk
• Fixes repo discovery in worktree_root_for_path, has_intervening_git_dir, and find_repository_for_file
• Unit tests for is_valid_git_dir (rejects empty dir, accepts dir with HEAD, accepts .git file, rejects nonexistent)
• Unit tests for worktree_root_for_path skipping empty .git dirs

Closes #1415

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