fix(i18n): exclude 404 pages from sitemap

Custom 404 pages (root and zh-cn) were indexed as regular content in
the sitemap. Add a filter to the @astrojs/sitemap integration that
matches the /404 path segment precisely via regex on the URL pathname.

website/astro.config.mjs

@@ -36,7 +36,11 @@ export default defineConfig({
   },
 
   integrations: [
-    sitemap(),
+    // Exclude custom 404 pages (all locales) from the sitemap — they are
+    // treated as normal content docs by Starlight even with disable404Route.
+    sitemap({
+      filter: (page) => !/\/404(\/|$)/.test(new URL(page).pathname),
+    }),
     starlight({
       title: 'BMAD Method',
       tagline: 'AI-driven agile development with specialized agents and workflows that scale from bug fixes to enterprise platforms.',