- SkillSourceConfig type + BUILT_IN_SOURCES (3 sources: anthropic, schwepps, daymade)
- fetchAllSources() fetches from anthropic-marketplace and github-tree source types
- parseSkillFrontmatter() extracts name/description from SKILL.md YAML blocks
- Idempotency: checks version exists before fetching, skips re-download on same SHA
- Caches SKILL.md to skills/cache/<skill-id>/<sha>/SKILL.md on disk
- Inserts skills, skill_versions, and skill_files rows into registry.db
- All 7 tests passing (TDD GREEN)
