ci: add minimal validation for SKILL.md required frontmatter

[buzuweidao]

What changed

Added a minimal CI guard for skill metadata quality:

• scripts/validate-skills-frontmatter.sh
• .github/workflows/validate-skills.yml

The check validates that every skills/**/SKILL.md includes frontmatter and these required fields:

• name
• description
• metadata
• metadata.version
• metadata.author

Why

This repository has high PR volume. A lightweight automated check catches common formatting omissions before manual review.

Scope

Process/CI only. No API/runtime behavior changes.

[dimitrisn442]

Can you add a path filter to the workflow to skip unnecessary runs on PRs that don't touch skills?

i.e.

on:                                                                                                               
  pull_request: 
    paths:
      - 'skills/**'
      - 'scripts/validate-skills-frontmatter.sh' 

[dimitrisn442]

Consider also validating the directory structure. Several open PRs place skills at skills/<skill-name>/ instead of the required skills/<namespace>/<skill-name>/.

i.e.

if [[ "$(echo "$file" | tr '/' '\n' | wc -l)" -lt 4 ]]; then
  echo "[ERROR] Skill must be under skills/<namespace>/<skill-name>/SKILL.md: $file"
  fail=1
fi