feat: add node picker dropdown to action node form for cross-references

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

frontend/src/components/tree-editor/NodeFormAction.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react'
 import { DynamicArrayField } from './DynamicArrayField'
-import { useTreeEditorStore } from '@/store/treeEditorStore'
+import { useTreeEditorStore, collectAllNodesFlat } from '@/store/treeEditorStore'
+import { Link2, X } from 'lucide-react'
 import { MarkdownContent } from '@/components/ui/MarkdownContent'
 import { InfoTip } from '@/components/common/InfoTip'
 import type { TreeStructure } from '@/types'
@@ -158,16 +159,65 @@ export function NodeFormAction({ node, onUpdate }: NodeFormActionProps) {
         />
       </div>
 
-      {/* Next step hint */}
-      {hasNextNode ? (
-        <p className="text-xs text-muted-foreground">
-          Next step is linked — click it on the canvas to edit.
+      {/* Link to existing node */}
+      <div>
+        <label className="flex items-center gap-1.5 text-sm font-medium text-foreground">
+          <Link2 className="h-3.5 w-3.5" />
+          Next Step
+        </label>
+        {hasNextNode ? (
+          <div className="mt-1 flex items-center gap-2 rounded-md border border-primary/30 bg-primary/5 px-3 py-2">
+            <span className="flex-1 truncate text-sm text-foreground">
+              Linked to: {(() => {
+                const treeStructure = useTreeEditorStore.getState().treeStructure
+                const allNodes = collectAllNodesFlat(treeStructure)
+                const target = allNodes.find(n => n.id === node.next_node_id)
+                return target ? target.label : node.next_node_id
+              })()}
+            </span>
+            <button
+              type="button"
+              onClick={() => onUpdate({ next_node_id: undefined })}
+              className="rounded p-1 text-muted-foreground hover:bg-accent hover:text-foreground"
+              title="Remove link"
+            >
+              <X className="h-3.5 w-3.5" />
+            </button>
+          </div>
+        ) : (
+          <select
+            value=""
+            onChange={(e) => {
+              if (e.target.value) {
+                onUpdate({ next_node_id: e.target.value })
+              }
+            }}
+            className={cn(
+              'mt-1 block w-full rounded-md border border-border px-3 py-2 text-sm',
+              'bg-card text-foreground',
+              'focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20'
+            )}
+          >
+            <option value="">Link to existing node...</option>
+            {(() => {
+              const treeStructure = useTreeEditorStore.getState().treeStructure
+              const allNodes = collectAllNodesFlat(treeStructure)
+              return allNodes
+                .filter(n => n.id !== node.id && n.type !== 'answer')
+                .map(n => (
+                  <option key={n.id} value={n.id}>
+                    {'  '.repeat(n.depth)}{n.type === 'decision' ? '?' : n.type === 'action' ? '>' : '*'} {n.label}
+                  </option>
+                ))
+            })()}
+          </select>
+        )}
+        <p className="mt-1 text-xs text-muted-foreground">
+          {hasNextNode
+            ? 'This action will navigate to the linked node.'
+            : 'Select a node to navigate to after this action, or save to create a new placeholder.'}
         </p>
-      ) : (
-        <p className="text-xs text-yellow-400/70">
-          Save to create a placeholder for the next step.
-        </p>
-      )}
+      </div>
     </div>
   )
 }

frontend/src/store/treeEditorStore.ts

@@ -123,6 +123,25 @@ export const findNodeInTree = (
   return null
 }
 
+/** Collect all nodes in the tree as a flat list with depth info. */
+export function collectAllNodesFlat(
+  root: TreeStructure | null
+): Array<{ id: string; label: string; type: string; depth: number }> {
+  if (!root) return []
+  const result: Array<{ id: string; label: string; type: string; depth: number }> = []
+
+  function walk(node: TreeStructure, depth: number) {
+    const label = node.type === 'decision'
+      ? (node.question || 'Untitled Decision')
+      : (node.title || `Untitled ${node.type}`)
+    result.push({ id: node.id, label, type: node.type, depth })
+    node.children?.forEach(child => walk(child, depth + 1))
+  }
+
+  walk(root, 0)
+  return result
+}
+
 // Helper to find parent of a node
 const findParentNode = (
   nodeId: string,