refactor(skill): move skill editor slice to core workflow store

Move SkillEditorSlice from injection pattern to core workflow store,
making it available to all workflow contexts (workflow-app, chatflow,
and future rag-pipeline).

- Add createSkillEditorSlice to core createWorkflowStore
- Remove complex type conversion logic from workflow-app/index.tsx
- Remove optional chaining (?.) and non-null assertions (!) from components
- Simplify slice composition with type assertions via unknown

web/app/components/workflow/skill/file-tree/tree-context-menu.tsx

@@ -22,7 +22,7 @@ const TreeContextMenu: FC<TreeContextMenuProps> = ({ treeRef }) => {
   const { data: treeData } = useSkillAssetTreeData()
 
   const handleClose = useCallback(() => {
-    storeApi.getState().setContextMenu?.(null)
+    storeApi.getState().setContextMenu(null)
   }, [storeApi])
 
   useClickAway(() => {