Add `maxRetries`, `retryDelayMs`, and `retryBackoff` to task config.
When a task fails and retries remain, the orchestrator waits with
exponential backoff and re-runs the task with a fresh agent conversation.
A `task_retry` event is emitted via `onProgress` for observability.
Cascade failure only occurs after all retries are exhausted.
Closes#30
JackChen