Support markdown response for coding agents like Claude Code and OpenCode.

Inspired by:

• feat(core): add HTTP markdown negotiation middleware fuma-nama/fumadocs#2408
• docs: support markdown response for AI better-auth/better-auth#5105
• https://x.com/bunjavascript/status/1971934734940098971
• https://x.com/mintlify/status/1972315377599447390
• sst/opencode@c148f10

Summary

Adds content negotiation middleware to automatically serve markdown when AI agents request documentation via the Accept header. No changes needed for browser users - HTML remains the default.

Changes

• Middleware: Detects Accept: text/markdown header and rewrites /docs/* to /llm/*
• Dependencies: Upgrade fumadocs-core to 15.8.3 for negotiation API
• Backward Compatible: Existing .md extension routes still work, browsers unaffected

How It Works

// middleware.ts
if (isMarkdownPreferred(request)) {
  // Rewrite /docs/components/button → /llm/components/button
  return NextResponse.rewrite(new URL(result, request.nextUrl))
}

For AI Agents:

GET /docs/components/button
Accept: text/markdown
→ Automatically serves markdown (no .md extension needed)

For Browsers:

GET /docs/components/button
Accept: text/html
→ Serves HTML as usual

Testing

# Test with curl
curl -H "Accept: text/markdown" https://ui.shadcn.com/docs/components/button

This makes shadcn/ui docs more accessible to AI coding assistants without affecting the user experience for human visitors.