feat(FN-1053): add AI prompt executor for cron workflow steps

- Define AiPromptExecutor type and update CronRunner constructor to accept injected executor
- Implement executeAiPromptStep with real agent session execution in worktrees
- Create createAiPromptStepExecutor factory and wire up in dashboard CLI
- Add comprehensive tests for AI prompt step execution (mock agent, errors, output capture)
- Fix mock typing for dashboard test build compatibility

packages/cli/src/commands/__tests__/dashboard.test.ts

@@ -122,6 +122,7 @@ vi.mock("@fusion/engine", async (importOriginal) => {
     })),
     scanIdleWorktrees: vi.fn().mockResolvedValue([]),
     cleanupOrphanedWorktrees: vi.fn().mockResolvedValue(0),
+    createAiPromptExecutor: vi.fn().mockResolvedValue(vi.fn().mockResolvedValue("mock AI response")),
   };
 });