Add JSON Schema for SKILL.md frontmatter validation

[haluke1]

Description

Create a JSON Schema that can validate the YAML frontmatter in SKILL.md files.

Requirements

• Validate all required fields (name, version, description, author)
• Validate capability values are from allowed set
• Validate version follows semver
• Validate dependency version constraints

Files to create

• schema/skill-frontmatter.json
• Update verify.sh to optionally validate against schema

Labels

good first issue, enhancement