[FEATURE] Add automated skill validation workflow

[gnoviawan]

Currently there is no CI/CD workflow to validate skills before merging. We need:

Requirements:

• YAML frontmatter validation
• Markdown linting (MD040 - fenced code blocks)
• Skill discovery test
• Dependency graph validation

Acceptance Criteria:

• GitHub Actions workflow created
• Runs on every PR
• Blocks merge on validation failure
• Provides clear error messages

Priority: Medium - quality assurance