fix(ai): import useMemo (missing since hand-off note refactor)

The handoffNote useMemo I added in the agent-history work referenced
useMemo without importing it. Vite was happy at compile time (lazy
binding) but the page crashes at first paint with "Can't find
variable: useMemo".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/ai.tsx

@@ -1,6 +1,7 @@
 import {
   useCallback,
   useEffect,
+  useMemo,
   useRef,
   useState,
 } from "react"