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/graph-view/hooks/use-graph-nodes.ts

@@ -24,6 +24,7 @@ export interface TaskNodeData extends Feature {
   onStartTask?: () => void;
   onStopTask?: () => void;
   onResumeTask?: () => void;
+  onSpawnTask?: () => void;
 }
 
 export type TaskNode = Node<TaskNodeData, 'task'>;
@@ -40,6 +41,7 @@ export interface NodeActionCallbacks {
   onStartTask?: (featureId: string) => void;
   onStopTask?: (featureId: string) => void;
   onResumeTask?: (featureId: string) => void;
+  onSpawnTask?: (featureId: string) => void;
 }
 
 interface UseGraphNodesProps {
@@ -112,6 +114,9 @@ export function useGraphNodes({
           onResumeTask: actionCallbacks?.onResumeTask
             ? () => actionCallbacks.onResumeTask!(feature.id)
             : undefined,
+          onSpawnTask: actionCallbacks?.onSpawnTask
+            ? () => actionCallbacks.onSpawnTask!(feature.id)
+            : undefined,
         },
       };