fix: validate skill frontmatter as yaml

[kaizhou-lab]

Summary

• Validate skill frontmatter with serde_yaml during import/read paths instead of hand-parsing name/description prefixes.
• Tighten SKILL.md frontmatter fence extraction and add regression coverage for invalid YAML, quoted descriptions, block scalars, and embedded builtin skills.
• Format invalid builtin skill descriptions as folded YAML scalars.

Test Plan

• cargo test -p aionui-extension
• cargo clippy -p aionui-extension -- -D warnings
• cargo fmt --all -- --check
• just push -u origin fix-skill-yaml-frontmatter