feat: Add task dependencies and spawn sub-task functionality

- Add edge dragging to create dependencies in graph view
- Add spawn sub-task action available in graph view and kanban board
- Implement ancestor context selection when spawning tasks
- Add dependency validation (circular, self, duplicate prevention)
- Include ancestor context in spawned task descriptions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/ui/src/components/views/board-view/shared/index.ts

@@ -6,3 +6,4 @@ export * from './testing-tab-content';
 export * from './priority-selector';
 export * from './branch-selector';
 export * from './planning-mode-selector';
+export * from './ancestor-context-section';