fix: reset scaffold trigger guard when wizard returns to foundation phase

hasTriggeredScaffold ref was not reset on Start Over/close, preventing
scaffold() from firing on subsequent attempts after an error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ai-builder/AIFlowBuilderModal.tsx

@@ -37,6 +37,11 @@ export function AIFlowBuilderModal({ isOpen, onClose }: AIFlowBuilderModalProps)
   // Auto-trigger scaffold after conversation starts (ref prevents double-fire)
   const hasTriggeredScaffold = useRef(false)
   useEffect(() => {
+    // Reset guard when wizard resets to foundation (Start Over or close)
+    if (phase === 'foundation') {
+      hasTriggeredScaffold.current = false
+      return
+    }
     if (phase === 'scaffolding' && !hasTriggeredScaffold.current && !useAIFlowBuilderStore.getState().suggestedBranches.length) {
       hasTriggeredScaffold.current = true
       scaffold()