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-derived-state.md

@@ -0,0 +1,29 @@
+---
+title: Subscribe to Derived State
+impact: MEDIUM
+impactDescription: reduces re-render frequency
+tags: rerender, derived-state, media-query, optimization
+---
+
+## Subscribe to Derived State
+
+Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.
+
+**Incorrect (re-renders on every pixel change):**
+
+```tsx
+function Sidebar() {
+  const width = useWindowWidth(); // updates continuously
+  const isMobile = width < 768;
+  return <nav className={isMobile ? "mobile" : "desktop"} />;
+}
+```
+
+**Correct (re-renders only when boolean changes):**
+
+```tsx
+function Sidebar() {
+  const isMobile = useMediaQuery("(max-width: 767px)");
+  return <nav className={isMobile ? "mobile" : "desktop"} />;
+}
+```