Gate Obsidian Toolbox behind TOOLBOX_VISIBLE flag

Toolbox is still being refined; hide it from clients on prod while keeping it
usable on staging. Entry points (header link, dashboard card) render only when
TOOLBOX_VISIBLE=true; a tools/layout redirects /tools/* to /dashboard otherwise.
Routes and ToolUsage table are untouched (no destructive migration).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/(student)/dashboard/page.tsx

@@ -168,9 +168,11 @@ export default async function StudentDashboard() {
         </div>
       )}
 
-      <div className="mt-8">
-        <ToolboxPromoCard />
-      </div>
+      {process.env.TOOLBOX_VISIBLE === "true" && (
+        <div className="mt-8">
+          <ToolboxPromoCard />
+        </div>
+      )}
 
       {locked.length > 0 && (
         <div className="mt-10">