Convert internal vault functions from &str to &Path for vault_path
parameter (run_git, cache_path, git_head_hash, git_changed_files,
git_uncommitted_new_files, load_cache, write_cache, to_relative_path,
finalize_and_cache, update_same_commit, update_different_commit).
String-heavy args reduced from 67% to 47%.
Split 5 large test functions into smaller focused tests to eliminate
Large Assertion Blocks findings. Extract parse_test_entry() and
parse_big_project_rels() helpers to reduce test boilerplate.
Code Health: 7.78 -> 8.54
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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