feat(12-05): export buildDecomposePrompt from agent module

- Add buildDecomposePrompt to public exports

src/agent/index.ts

@@ -19,4 +19,9 @@ export { ClaudeAgentManager } from './manager.js';
 export { MockAgentManager, type MockAgentScenario } from './mock-manager.js';
 
 // Agent prompts
-export { buildDiscussPrompt, buildBreakdownPrompt, buildExecutePrompt } from './prompts.js';
+export {
+  buildDiscussPrompt,
+  buildBreakdownPrompt,
+  buildExecutePrompt,
+  buildDecomposePrompt,
+} from './prompts.js';