perf: optimize agent stream view rendering

- Separate streaming item from FlatList data array to prevent full
  re-diffs on every streaming chunk
- Memoize renderStreamItem with useCallback
- Memoize AgentThoughtMessage component with React.memo

The main fix is rendering the streaming message in ListHeaderComponent
instead of prepending it to the data array, which was causing the
entire list to re-diff on every character streamed.

packages/app/src/components/message.tsx

@@ -963,7 +963,7 @@ const agentThoughtStylesheet = StyleSheet.create((theme) => ({
   },
 }));
 
-export function AgentThoughtMessage({ message, status = "ready" }: AgentThoughtMessageProps) {
+export const AgentThoughtMessage = memo(function AgentThoughtMessage({ message, status = "ready" }: AgentThoughtMessageProps) {
   const { theme } = useUnistyles();
   const [isExpanded, setIsExpanded] = useState(false);
   const markdownContent = useMemo(() => message?.trim() ?? "", [message]);
@@ -1102,7 +1102,7 @@ export function AgentThoughtMessage({ message, status = "ready" }: AgentThoughtM
       isLoading={status !== "ready"}
     />
   );
-}
+});
 
 interface ToolCallProps {
   toolName: string;