Built-in SKILL.md description values contained unquoted ': ' sequences, which strict YAML parsers (e.g. Codex) reject — silently dropping the skill at load.
- Quote all eight built-in skill descriptions.
- ensureSkillFrontmatter() re-synthesizes frontmatter that has a name but fails YAML validation, so malformed imports are repaired instead of dropped.
- Unify frontmatter delimiter parsing into a single frontmatterParts helper.
- Add strict-YAML regression tests over the built-in skills, plus unit tests for the recovery branch and delimiter variants.
Closes#3851.
