fix: deploy middleware as Vercel Edge Function for content negotiation

The Astro middleware in src/middleware.ts was not running at request time
for pre-rendered (static) pages. The default middlewareMode ('classic')
only executes middleware at build time for static pages, so Accept:
text/markdown requests were served HTML from Vercel's CDN cache.

Setting middlewareMode: 'edge' deploys the middleware as a separate
Vercel Edge Function that runs at request time for ALL requests,
including static pages. This enables content negotiation so agents
sending Accept: text/markdown get clean markdown automatically.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: rachaelrenk

astro.config.mjs

@@ -163,5 +163,9 @@ export default defineConfig({
 		}),
 		docsMarkdownIntegration(),
 	],
-	adapter: vercel(),
+	// Deploy Astro middleware as a Vercel Edge Function so it runs at
+	// request time for ALL pages, including pre-rendered static pages.
+	// Without this, middleware only runs at build time for static pages
+	// and content negotiation (Accept: text/markdown) doesn't work.
+	adapter: vercel({ middlewareMode: 'edge' }),
 });