fix(oss-readiness): make frontmatter valid yaml

[bntvllnt]

Summary

• fix the oss-readiness skill frontmatter so it parses as valid YAML
• change compatibility from a plain scalar containing : to a block scalar
• keep the skill content unchanged apart from the frontmatter formatting fix

Root cause

• the frontmatter parser choked on Optional CLI helpers: inside an unquoted scalar
• that made the skill invalid for strict YAML frontmatter consumers, which is why it was missing from skills.sh indexing

Verification

• parsed the frontmatter with Python yaml.safe_load