docs: migrate page metadata from MDX to layout.tsx (#904)

Move JS metadata exports out of page.mdx files into per-directory
layout.tsx files. MDX files now contain pure markdown content, making
them render cleanly on GitHub without visible JS import/export lines.

No content changes — only the metadata mechanism is relocated.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

docs/src/app/commands/layout.tsx

@@ -0,0 +1,7 @@
+import { pageMetadata } from "@/lib/page-metadata";
+
+export const metadata = pageMetadata("commands");
+
+export default function Layout({ children }: { children: React.ReactNode }) {
+  return children;
+}

docs/src/app/commands/page.mdx

@@ -1,7 +1,3 @@
-import { pageMetadata } from "@/lib/page-metadata"
-
-export const metadata = pageMetadata("commands")
-
 # Commands
 
 ## Core