gray-matter uses yaml.safeLoad() which was removed in js-yaml 4.x, causing 500
errors on app store pages after the js-yaml 4.1.1 update (CWE-1321 fix)
- Add parseFrontmatter function using yaml.load with JSON_SCHEMA
- Add type guard for safe type narrowing
- Add unit tests for frontmatter parsing and security
- Remove gray-matter dependency