Add `docs/agent-skills.md` with installation instructions for multiple AI
coding agents including Claude Code, VS Code/Copilot, Gemini CLI, Cline,
and Goose. The guide provides three installation methods: Claude Code
plugin, Vercel Skills CLI (`npx skills add`), and manual installation.
Update `README.md` to link to the new guide and reframe the plugin
section around the broader "Agent Skills" concept.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
