Introduce a dedicated Zustand ArtifactSlice to manage artifact selection
state with mutual exclusion against the main file tree. Artifact files
from the sandbox can now be opened as tabs in the skill editor, rendered
via a lightweight ArtifactContentPanel that reuses ReadOnlyFilePreview.
- Add START_TAB_ID constant and StartTabItem/StartTabContent components
- Default to Start tab when no file tabs are open
- Optimize zustand selectors to subscribe to specific Map values instead of
entire Map objects, reducing unnecessary re-renders when other tabs change
- Refactor useSkillFileSave to accept precise values instead of Map/Set
Implement unified drag system that supports both internal node moves
and external file uploads with consistent UI feedback. Uses native
HTML5 drag API with shared visual states (isDragOver, isBlinking,
DragActionTooltip showing 'Move to' or 'Upload to').
- 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
