Add AI Virus Deflection story to homepage and new agent briefing page

Give the public homepage a themed breach-alert hero and add a /briefing
page with the full mission narrative, both linking into the existing
game dashboard flow.

src/Website/Controller/HomeController.php

@@ -15,4 +15,11 @@ final class HomeController extends AbstractController
         return $this->render(
             'website/home/index.html.twig');
     }
+
+    #[Route(path: '/briefing', name: 'website_intro')]
+    public function intro(): Response
+    {
+        return $this->render(
+            'website/home/intro.html.twig');
+    }
 }