Phase 1 of RFC #351 — define AgentFrontmatter type, parseFrontmatterMarkdown()
and validateFrontmatter() with 45 tests.
- Built-in minimal YAML parser (no new deps)
- Never throws on malformed input — degrades gracefully
- All fields use T | null (no optional properties)
Refs #351
Bundles must run without env vars — env vars are overrides, not requirements.
Single function: env(name, fallback) always returns string with a default.
- Removed requireEnv and optionalEnv
- Updated bundle entries, tests, and skill docs
小橘 🍊