feat(agent): add 24-hour cache for agent endpoint

The .agent endpoint uses AI generation (GPT-4o-mini) which is expensive.
Adding caching reduces API costs and improves response times for repeated
requests.

Cache settings:
- s-maxage=86400: CDN caches for 24 hours
- stale-while-revalidate=3600: Serve stale content for 1 hour while
  revalidating in the background

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomasdavis

apps/registry/pages/api/[username].js

@@ -43,6 +43,14 @@ export default async function handler(req, res) {
   );
   res.setHeader('Access-Control-Allow-Origin', '*');
 
+  // Add 24-hour cache for agent endpoint (AI-generated, expensive)
+  if (extension === 'agent') {
+    res.setHeader(
+      'Cache-Control',
+      'public, s-maxage=86400, stale-while-revalidate=3600'
+    );
+  }
+
   // if there is an extension, render the file as if it were present
   // some formatters require special response headers
   headers.forEach((header) => {