Fix modal draft state overwriting store-managed children

Problem: Child nodes created via NodePicker while editing a parent node
would disappear when clicking "Done" on the modal. This caused:
- Child nodes not appearing in tree after closing modal
- Validation errors about non-existent nodes
- Tree unable to save

Root cause: Modal used structuredClone() to create local draft state,
which included a stale `children: []` array. When saving, this overwrote
the actual children that were added to the store via addNode().

Fix: Exclude `children` from the draft when saving, since children are
managed separately by addNode/deleteNode store actions.

Also documented this critical pattern in LESSONS-LEARNED.md for future
reference when implementing modals with local draft state.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -31,7 +31,10 @@ export function NodeEditorModal({ node, onClose, isNewNode = false }: NodeEditor
 
   const handleSave = () => {
     // Commit all draft changes to the store
-    updateNode(node.id, draft)
+    // IMPORTANT: Exclude 'children' from the update - children are managed separately
+    // by addNode/deleteNode and we don't want to overwrite them with stale draft data
+    const { children, ...draftWithoutChildren } = draft
+    updateNode(node.id, draftWithoutChildren)
     onClose()
   }