Add CONTRIBUTING.md guide for skill submissions

[DeanKelly751]

Summary

There is no contributing guide, making it unclear how others should structure and submit new skills.

Proposed Solution

Add a CONTRIBUTING.md that covers:

• Skill directory structure (skill-name/SKILL.md)
• Required YAML frontmatter fields (name, description)
• Description best practices
• How to test a skill locally
• PR process

Acceptance Criteria

• CONTRIBUTING.md exists at repo root
• Covers skill structure, naming conventions, and submission process
• README links to the contributing guide