fix(seo): noindex staging builds by default

- add PUBLIC_ALLOW_INDEXING as an explicit production indexing opt-in
- emit robots meta noindex,nofollow unless indexing is enabled
- generate robots.txt from the same indexing flag
- document staging SEO behavior for migration preview hosts

www/src/pages/robots.txt.ts

@@ -0,0 +1,23 @@
+const allowIndexing = import.meta.env.PUBLIC_ALLOW_INDEXING === 'true';
+
+const body = allowIndexing
+  ? [
+      'User-agent: *',
+      'Allow: /',
+      'Sitemap: https://www.azinstitute4autism.com/sitemap.xml',
+      ''
+    ].join('\n')
+  : [
+      'User-agent: *',
+      'Allow: /',
+      '# Staging and migration builds emit meta robots noindex,nofollow.',
+      ''
+    ].join('\n');
+
+export function GET() {
+  return new Response(body, {
+    headers: {
+      'Content-Type': 'text/plain; charset=utf-8'
+    }
+  });
+}