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/engine/src/index.ts

@@ -14,7 +14,7 @@ export { withRateLimitRetry } from "./rate-limit-retry.js";
 export { PrMonitor, type PrComment, type TrackedPr, type OnNewCommentsCallback } from "./pr-monitor.js";
 export { PrCommentHandler } from "./pr-comment-handler.js";
 export { NtfyNotifier, type NtfyNotifierOptions } from "./notifier.js";
-export { CronRunner, type CronRunnerOptions } from "./cron-runner.js";
+export { CronRunner, type CronRunnerOptions, type AiPromptExecutor, createAiPromptExecutor } from "./cron-runner.js";
 export { StuckTaskDetector, type StuckTaskDetectorOptions, type DisposableSession } from "./stuck-task-detector.js";
 export { TokenCapDetector, type TokenCapCheckResult } from "./token-cap-detector.js";
 export { SelfHealingManager, type SelfHealingOptions } from "./self-healing.js";