feat(KB-206): add stuck task detection and auto-recovery

- Add taskStuckTimeoutMs setting to detect tasks with no activity
- Create StuckTaskDetector to poll in-progress tasks every 30s
- Integrate heartbeat tracking into executor via step callbacks
- Implement recovery flow: abort stuck sessions, retry with preserved progress
- Export detector and wire into dashboard for real-time monitoring
- Add comprehensive tests and documentation to AGENTS.md

packages/engine/src/index.ts

@@ -13,3 +13,4 @@ export { PrMonitor, type PrComment, type TrackedPr, type OnNewCommentsCallback }
 export { PrCommentHandler } from "./pr-comment-handler.js";
 export { NtfyNotifier, type NtfyNotifierOptions } from "./notifier.js";
 export { CronRunner, type CronRunnerOptions } from "./cron-runner.js";
+export { StuckTaskDetector, type StuckTaskDetectorOptions, type DisposableSession } from "./stuck-task-detector.js";