fix(agents): add YAML frontmatter to AGENTS.md

[JasonOA888]

Fixes #79

Problem

gemini-cli extensions install fails with:

Invalid agent definition: Missing mandatory YAML frontmatter.
Agent Markdown files MUST start with YAML frontmatter enclosed
in triple-dashes "---" (e.g., ---\nname: my-agent\n---).

Fix

Added required YAML frontmatter:

---
name: huggingface-skills
description: Hugging Face Agent Skills - specialized instructions and domain-specific knowledge for AI agents
---

Impact

• ✅ gemini-cli can now successfully install huggingface-skills extension
• ✅ AGENTS.md is now valid agent definition format
• ✅ No functional changes to skills content