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/utils/dependency-validation.ts

@@ -0,0 +1,51 @@
+import { Feature } from '@/store/app-store';
+
+/**
+ * Checks if adding a dependency from sourceId to targetId would create a circular dependency.
+ * Uses DFS to detect if targetId can reach sourceId through existing dependencies.
+ *
+ * @param features - All features in the system
+ * @param sourceId - The feature that would become a dependency (the prerequisite)
+ * @param targetId - The feature that would depend on sourceId
+ * @returns true if adding this dependency would create a cycle
+ */
+export function wouldCreateCircularDependency(
+  features: Feature[],
+  sourceId: string,
+  targetId: string
+): boolean {
+  const featureMap = new Map(features.map((f) => [f.id, f]));
+  const visited = new Set<string>();
+
+  function canReach(currentId: string, targetId: string): boolean {
+    if (currentId === targetId) return true;
+    if (visited.has(currentId)) return false;
+
+    visited.add(currentId);
+    const feature = featureMap.get(currentId);
+    if (!feature?.dependencies) return false;
+
+    for (const depId of feature.dependencies) {
+      if (canReach(depId, targetId)) return true;
+    }
+    return false;
+  }
+
+  // Check if source can reach target through existing dependencies
+  // If so, adding target -> source would create a cycle
+  return canReach(sourceId, targetId);
+}
+
+/**
+ * Checks if a dependency already exists between two features.
+ *
+ * @param features - All features in the system
+ * @param sourceId - The potential dependency (prerequisite)
+ * @param targetId - The feature that might depend on sourceId
+ * @returns true if targetId already depends on sourceId
+ */
+export function dependencyExists(features: Feature[], sourceId: string, targetId: string): boolean {
+  const targetFeature = features.find((f) => f.id === targetId);
+  if (!targetFeature?.dependencies) return false;
+  return targetFeature.dependencies.includes(sourceId);
+}