chore: vendor react-best-practices agent skill for cursor agents

Adds the Vercel skill under .agents with famapp path patterns.

Includes skills-lock.json pin.

.agents/skills/react-best-practices/rules/rendering-activity.md

@@ -0,0 +1,26 @@
+---
+title: Use Activity Component for Show/Hide
+impact: MEDIUM
+impactDescription: preserves state/DOM
+tags: rendering, activity, visibility, state-preservation
+---
+
+## Use Activity Component for Show/Hide
+
+Use React's `<Activity>` to preserve state/DOM for expensive components that frequently toggle visibility.
+
+**Usage:**
+
+```tsx
+import { Activity } from "react";
+
+function Dropdown({ isOpen }: Props) {
+  return (
+    <Activity mode={isOpen ? "visible" : "hidden"}>
+      <ExpensiveMenu />
+    </Activity>
+  );
+}
+```
+
+Avoids expensive re-renders and state loss.