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/upstream/rules/rerender-transitions.md

@@ -0,0 +1,40 @@
+---
+title: Use Transitions for Non-Urgent Updates
+impact: MEDIUM
+impactDescription: maintains UI responsiveness
+tags: rerender, transitions, startTransition, performance
+---
+
+## Use Transitions for Non-Urgent Updates
+
+Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.
+
+**Incorrect (blocks UI on every scroll):**
+
+```tsx
+function ScrollTracker() {
+  const [scrollY, setScrollY] = useState(0);
+  useEffect(() => {
+    const handler = () => setScrollY(window.scrollY);
+    window.addEventListener("scroll", handler, { passive: true });
+    return () => window.removeEventListener("scroll", handler);
+  }, []);
+}
+```
+
+**Correct (non-blocking updates):**
+
+```tsx
+import { startTransition } from "react";
+
+function ScrollTracker() {
+  const [scrollY, setScrollY] = useState(0);
+  useEffect(() => {
+    const handler = () => {
+      startTransition(() => setScrollY(window.scrollY));
+    };
+    window.addEventListener("scroll", handler, { passive: true });
+    return () => window.removeEventListener("scroll", handler);
+  }, []);
+}
+```