feat(rah-light): remove chat pane, simplify to two-panel layout

- Deleted src/components/panes/ChatPane.tsx
- Removed 'chat' from PaneType in types.ts
- Updated ThreePanelLayout.tsx to remove chat pane case and references
- Updated LeftToolbar.tsx to remove chat icon and type
- Default slot B is now null (closed) instead of chat
- Cmd+\ now opens node pane instead of chat

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

src/components/panes/index.ts

@@ -1,5 +1,4 @@
 export { default as NodePane } from './NodePane';
-export { default as ChatPane } from './ChatPane';
 export { default as WorkflowsPane } from './WorkflowsPane';
 export { default as DimensionsPane } from './DimensionsPane';
 export { default as MapPane } from './MapPane';