fix: wire AI-generated flow structures into editor stores

The useEditorAI hook was ignoring result.working_tree from AI responses,
so generated steps/trees never appeared in the editor. Now:
- useEditorAI calls onFlowUpdate when working_tree is present in response
- ProceduralEditorPage handles steps + intake form updates via replaceSteps
- TreeEditorPage handles tree structure updates via replaceTreeStructure
- Both stores have new bulk-replace methods for AI integration

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

frontend/src/store/treeEditorStore.ts

@@ -283,6 +283,9 @@ interface TreeEditorState {
   syncMarkdownToTree: () => void
   syncTreeToMarkdown: () => void
 
+  // Actions - AI Integration
+  replaceTreeStructure: (structure: TreeStructure) => void
+
   // Actions - State
   setLoading: (loading: boolean) => void
   setSaving: (saving: boolean) => void
@@ -1015,6 +1018,15 @@ export const useTreeEditorStore = create<TreeEditorState>()(
         }
       },
 
+      // AI Integration
+      replaceTreeStructure: (structure: TreeStructure) => {
+        set((state) => {
+          state.treeStructure = structure
+          state.selectedNodeId = structure.id
+          state.isDirty = true
+        })
+      },
+
       setLoading: (loading: boolean) => {
         set((state) => { state.isLoading = loading })
       },