yaml: unmarshal errors

[Liripo]

example skills is: https://github.com/ClawBio/ClawBio/blob/main/skills/affinity-proteomics/SKILL.md

$ skill-validator score evaluate --provider claude-cli affinity-proteomics --skill-only
Error: loading skill: parsing frontmatter YAML: yaml: unmarshal errors:
  line 9: cannot unmarshal !!seq into string
  line 18: cannot unmarshal !!map into string

[dacharyc]

Ahh, thanks for the issue. The SKILL.md you've linked here contains some metadata that doesn't match the spec; the spec calls for: "a map from string keys to string values." But we were crashing on load instead of handling the invalid YAML and reporting it, so I've added some updates in #93 that should handle the crash. You'll still see the metadata values as invalid, though.