refactor(skill): unify root/blank constants and eliminate magic strings

- Add constants.ts with ROOT_ID, CONTEXT_MENU_TYPE, NODE_MENU_TYPE
- Add root utilities to tree-utils.ts (isRootId, toApiParentId, etc.)
- Replace '__root__' with ROOT_ID for consistent root identifier
- Replace inline 'blank'/'root' strings with constants
- Use NodeMenuType for type-safe menu type props
- Remove duplicate ContextMenuType from types.ts, use from constants.ts

web/app/components/workflow/skill/hooks/use-file-drop.ts

@@ -9,6 +9,7 @@ import { useStore as useAppStore } from '@/app/components/app/store'
 import Toast from '@/app/components/base/toast'
 import { useWorkflowStore } from '@/app/components/workflow/store'
 import { useCreateAppAssetFile } from '@/service/use-app-asset'
+import { ROOT_ID } from '../constants'
 
 type FileDropTarget = {
   folderId: string | null
@@ -32,8 +33,8 @@ export function useFileDrop() {
 
     e.dataTransfer.dropEffect = 'copy'
 
-    // Use '__root__' to indicate dragging over root (to distinguish from "not dragging")
-    storeApi.getState().setDragOverFolderId(target.folderId ?? '__root__')
+    // Use ROOT_ID to indicate dragging over root (to distinguish from null = "not dragging")
+    storeApi.getState().setDragOverFolderId(target.folderId ?? ROOT_ID)
   }, [storeApi])
 
   const handleDragLeave = useCallback((e: React.DragEvent) => {

web/app/components/workflow/skill/hooks/use-file-operations.ts

@@ -7,6 +7,7 @@ import type { NodeApi, TreeApi } from 'react-arborist'
 import type { TreeNodeData } from '../type'
 import { useStore as useAppStore } from '@/app/components/app/store'
 import { useWorkflowStore } from '@/app/components/workflow/store'
+import { toApiParentId } from '../utils/tree-utils'
 import { useCreateOperations } from './use-create-operations'
 import { useModifyOperations } from './use-modify-operations'
 import { useSkillAssetTreeData } from './use-skill-asset-tree'
@@ -31,7 +32,7 @@ export function useFileOperations({
   const storeApi = useWorkflowStore()
   const { data: treeData } = useSkillAssetTreeData()
 
-  const parentId = nodeId === 'root' ? null : nodeId
+  const parentId = toApiParentId(nodeId)
 
   const createOps = useCreateOperations({
     parentId,