docs(AddAgentModal): add comment explaining modal button pattern

Add explanatory comment for combining Button and Modal components in Hero UI pattern to clarify focus management requirements

src/renderer/src/components/Popups/AddAgentModal.tsx

@@ -220,6 +220,9 @@ export const AddAgentModal: React.FC = () => {
 
   return (
     <ErrorBoundary>
+      {/* NOTE: Hero UI Modal Pattern: Combine the Button and Modal components into a single
+      encapsulated component. This is because the Modal component needs to bind the onOpen
+      event handler to the Button for proper focus management.  */}
       <Button onPress={onOpen} className="justify-start bg-transparent text-foreground-500 hover:bg-accent">
         <Plus size={16} style={{ marginRight: 4, flexShrink: 0 }} />
         {t('agent.add.title')}