feat(KB-040): add break-into-subtasks toggle for task creation

- Add breakIntoSubtasks flag to core Task model and persist to store
- Plumb subtask flag through dashboard API routes with validation
- Add subtask toggle UI to inline task creation card
- Update triage agent to handle automatic subtask breakdown when enabled
- Add comprehensive tests for store, API routes, and UI components
- Include changeset for patch release documenting the new feature

packages/core/src/store.ts

@@ -193,6 +193,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       description: input.description,
       column: input.column || "triage",
       dependencies: input.dependencies || [],
+      breakIntoSubtasks: input.breakIntoSubtasks === true ? true : undefined,
       steps: [],
       currentStep: 0,
       log: [{ timestamp: now, action: "Task created" }],