fix: generate robots.txt dynamically from site base URL

Replace static robots.txt with an Astro endpoint that uses the
configured site URL, so sitemap references are correct on both
fork deployments and production.

website/src/pages/robots.txt.ts

@@ -1,5 +1,10 @@
-# BMAD Method Documentation
-# https://docs.bmad-method.org/
+import type { APIRoute } from 'astro';
+
+export const GET: APIRoute = ({ site }) => {
+  const siteUrl = site?.href.replace(/\/$/, '') ?? '';
+
+  const body = `# BMAD Method Documentation
+# ${siteUrl}/
 #
 # This file controls web crawler access to the documentation site.
 
@@ -34,4 +39,10 @@ User-agent: cohere-ai
 Allow: /
 
 # Sitemap
-Sitemap: https://docs.bmad-method.org/sitemap-index.xml
+Sitemap: ${siteUrl}/sitemap-index.xml
+`;
+
+  return new Response(body, {
+    headers: { 'Content-Type': 'text/plain; charset=utf-8' },
+  });
+};