Fix skill frontmatter YAML

[songxuchen]

What changed

• Quote compatibility frontmatter values that contain credentials: ... so GitHub and YAML parsers can read the skill metadata.
• Add a validator check for unquoted : in flat frontmatter values so this class of issue is caught in CI.

Why

GitHub was showing mapping values are not allowed in this context for video-to-music/SKILL.md because YAML plain scalars cannot contain : unless the value is quoted or folded. The same pattern existed in several Sonilo skill files.

Validation

• python3 tests/validate.py
• python3 + PyYAML parse of every */SKILL.md frontmatter
• git diff --check

[sapient-app]

Lightsage docs evals

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Commit: 8685b97
Status: waiting for staging docs URL