nook: fix the landing title, isolate /blog, grow the sitemap

The landing page spent its strongest on-page signal on the word "Home";
the title now carries the positioning line and the OG title carries the
full brand phrase. /blog on a product subdomain served the personal blog
in dev (vinxi ignores the vercel.json rewrites that protect production),
splitting the brand and crawl equity — subdomain hits now render the
shared 404. /agents, /changelog and /faq join the sitemap registry so
they are indexed as they land.

Tests: bun test (539 pass); vinxi build clean, no new tsc errors.

Issues: FRE-8, FRE-15, FRE-9

src/routes/blog/[title]/index.tsx

@@ -21,6 +21,8 @@ import { api } from "~/lib/api";
 import CustomScrollbar from "~/components/CustomScrollbar";
 import "../post.css";
 import { Post } from "~/db/types";
+import { useSite } from "~/context/SiteContext";
+import NotFound from "../../[...404]";
 
 const getPostByTitle = query(
   async (
@@ -262,6 +264,7 @@ const getPostByTitle = query(
 );
 
 export default function PostPage() {
+  const site = useSite();
   const params = useParams();
   const [searchParams] = useSearchParams();
 
@@ -297,6 +300,12 @@ export default function PostPage() {
     return str.includes('data-type="mermaid"');
   };
 
+  // Host-dispatch guard: blog posts live on the apex domain only (see the
+  // guard in blog/index.tsx for the dev-server rationale).
+  if (site().id !== "main") {
+    return <NotFound />;
+  }
+
   return (
     <Show
       when={data()}