feat(12-03): extend AgentStoppedEvent reason with decompose_complete

- Add 'decompose_complete' to AgentStoppedEvent reason type union
- Follows pattern established in Phase 11 for context_complete/breakdown_complete

src/events/types.ts

@@ -158,7 +158,8 @@ export interface AgentStoppedEvent extends DomainEvent {
       | 'error'
       | 'waiting_for_input'
       | 'context_complete'
-      | 'breakdown_complete';
+      | 'breakdown_complete'
+      | 'decompose_complete';
   };
 }