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/kanban-board.tsx

@@ -41,6 +41,7 @@ interface KanbanBoardProps {
   onImplement: (feature: Feature) => void;
   onViewPlan: (feature: Feature) => void;
   onApprovePlan: (feature: Feature) => void;
+  onSpawnTask?: (feature: Feature) => void;
   featuresWithContext: Set<string>;
   runningAutoTasks: string[];
   shortcuts: ReturnType<typeof useKeyboardShortcutsConfig>;
@@ -73,6 +74,7 @@ export function KanbanBoard({
   onImplement,
   onViewPlan,
   onApprovePlan,
+  onSpawnTask,
   featuresWithContext,
   runningAutoTasks,
   shortcuts,
@@ -184,6 +186,7 @@ export function KanbanBoard({
                         onImplement={() => onImplement(feature)}
                         onViewPlan={() => onViewPlan(feature)}
                         onApprovePlan={() => onApprovePlan(feature)}
+                        onSpawnTask={() => onSpawnTask?.(feature)}
                         hasContext={featuresWithContext.has(feature.id)}
                         isCurrentAutoTask={runningAutoTasks.includes(feature.id)}
                         shortcutKey={shortcutKey}