fix: validate nested skill frontmatter

[Damin-Lee]

Summary

Fix invalid YAML frontmatter in OpenClaw skill sources and extend maintenance checks so nested openclaw/skills/*/SKILL.md files are validated as part of skill:check and static tests.

Root Cause

openclaw/skills/*/SKILL.md files were not included in nested skill discovery, so maintenance checks never parsed their frontmatter. Three descriptions also used plain YAML scalars containing : , which Codex rejected as invalid mappings.

What Changed

• Quote the description: frontmatter values in the affected OpenClaw skill source files.
• Make discoverSkillFiles() recurse into nested skill directories.
• Add lightweight frontmatter validation that flags unquoted scalar values containing : .
• Surface frontmatter validation failures in skill:check.
• Add tests covering invalid frontmatter and all discovered SKILL.md files.

Verification

• bun test test/skill-parser.test.ts test/skill-validation.test.ts
• bun run skill:check
  • Still reports pre-existing generated-host freshness/missing issues, but no OpenClaw frontmatter YAML errors remain.

[Damin-Lee]

Closing this as a duplicate of #834. I moved the nested skill-maintenance coverage onto the existing OpenClaw YAML PR branch so review stays consolidated in one place.