parse_md_file: extract infer_type_from_folder(), resolve_is_a(),
parse_created_at(), extract_fm_and_rels(), read_file_metadata().
Reduces from 83 LoC to ~25 LoC thin orchestration.
git_changed_files: extract collect_md_paths_from_diff() and
collect_md_paths_from_porcelain() as iterator-based helpers.
Eliminates Bumpy Road (2 bumps -> 0) and Deep Nesting (4 levels -> 2).
Code Health: 6.35 -> 6.92
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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