Use YAML parser for richer skill frontmatter

[dieterbaier]

Summary

Follow-up from PR #51 re-review: the current scripts/build-agent-adapters.js frontmatter parser is intentionally simple and sufficient for flat metadata such as name and adapter_expose.

If generated adapter behavior later depends on richer or nested skill metadata, replace the ad hoc frontmatter parsing with a real YAML parser.

Context

PR #51 introduced generated agent adapters and simple SKILL.md metadata filtering. The reviewer marked this as non-blocking for the PR, but worth tracking so it is not forgotten.

Acceptance Criteria

• Decide whether richer/nested skill metadata is needed.
• If needed, use a YAML parser for SKILL.md frontmatter in adapter generation.
• Keep the generator small and deterministic.
• Preserve node scripts/check-agent-adapters.js behavior for CI.