fix: add YAML frontmatter to agents/AGENTS.md

[shivam8764]

AGENTS.md is missing the YAML frontmatter block that Gemini CLI requires to load the extension properly. Without it, installing the extension throws:

Error loading agent from huggingface-skills: Invalid agent definition: Missing mandatory YAML frontmatter

and anyone who patches it locally gets the broken version pulled back every time they run gemini extensions update.

just adds the frontmatter at the top of the file, nothing else changes.

same fix as #78 which has been sitting open for a while — closes #121 and #79