Wrap @theme-original/Root so the markdown plugin keeps its injection

src/theme/Root.tsx

@@ -1,4 +1,5 @@
 import React, { type ReactNode } from 'react';
+import Root from '@theme-original/Root';
 
 import ForAgentsPanel from '@site/src/components/ForAgentsPanel';
 
@@ -10,12 +11,18 @@ interface Props {
  * `Root` wraps the whole app above the router, so anything rendered here
  * survives client-side navigation. That is what the "For agents" panel needs:
  * one instance for the entire site rather than one per page (#2586).
+ *
+ * This must WRAP `@theme-original/Root`, not replace it:
+ * `docusaurus-markdown-source-plugin` ships its own Root, which injects the
+ * "Open Markdown" dropdown and the hash-scroll behavior on docs pages. A
+ * user-land Root shadows it outright, which is exactly the regression that
+ * removed the button site-wide (#2712).
  */
-export default function Root({ children }: Props): ReactNode {
+export default function RootWrapper({ children }: Props): ReactNode {
   return (
-    <>
+    <Root>
       {children}
       <ForAgentsPanel />
-    </>
+    </Root>
   );
 }