Add Agent Skills installation guide

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>

typedoc.config.mjs

@@ -10,6 +10,7 @@ const config = {
   projectDocuments: [
     "docs/overview.md",
     "docs/quickstart.md",
+    "docs/agent-skills.md",
     "docs/patterns.md",
     "docs/migrate_from_openai_apps.md",
   ],