chore(cc-check): improve skill quality toward production-ready

[philoserf]

Summary

• Remove deprecated "commands" type from SKILL.md and all reference files
• Add TOC to common-failures.md (220+ lines, per project conventions)
• Remove invalid name/description frontmatter from reference and asset files
• Add read-only vs active testing decision table and test priority heuristic
• Update stale hook example (validate_config.py → auto-format.sh)
• Rename "Portability" test to "Frontmatter Validation" for clarity
• Broaden trigger description with additional synonyms
• Clarify output directory guidance (inline default, mkdir -p for persistence)

Quality Score

• Before: 3.72 (Good)
• After: ~4.4+ (Good → Production Ready)

Test plan

• Verify all markdown links resolve
• Run bunx prettier --check on changed files
• Invoke /cc-check on a test skill to confirm workflow

Closes #306

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