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/advanced-use-latest.md

@@ -0,0 +1,39 @@
+---
+title: useEffectEvent for Stable Callback Refs
+impact: LOW
+impactDescription: prevents effect re-runs
+tags: advanced, hooks, useEffectEvent, refs, optimization
+---
+
+## useEffectEvent for Stable Callback Refs
+
+Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
+
+**Incorrect (effect re-runs on every callback change):**
+
+```tsx
+function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
+  const [query, setQuery] = useState("");
+
+  useEffect(() => {
+    const timeout = setTimeout(() => onSearch(query), 300);
+    return () => clearTimeout(timeout);
+  }, [query, onSearch]);
+}
+```
+
+**Correct (using React's useEffectEvent):**
+
+```tsx
+import { useEffectEvent } from "react";
+
+function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
+  const [query, setQuery] = useState("");
+  const onSearchEvent = useEffectEvent(onSearch);
+
+  useEffect(() => {
+    const timeout = setTimeout(() => onSearchEvent(query), 300);
+    return () => clearTimeout(timeout);
+  }, [query]);
+}
+```