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)/layout.tsx

@@ -57,9 +57,11 @@ export default async function StudentLayout({ children }: { children: React.Reac
           <Link href="/wrapped" className="text-sm hover:underline" style={{ color: "var(--muted-foreground)" }}>
             Wrapped
           </Link>
-          <Link href="/tools" className="text-sm hover:underline" style={{ color: "var(--muted-foreground)" }}>
-            Инструменты
-          </Link>
+          {process.env.TOOLBOX_VISIBLE === "true" && (
+            <Link href="/tools" className="text-sm hover:underline" style={{ color: "var(--muted-foreground)" }}>
+              Инструменты
+            </Link>
+          )}
           <Link href="/questions" className="text-sm hover:underline" style={{ color: "var(--muted-foreground)" }}>
             Вопросы
           </Link>