feat(orchestration): orchestrator DLQ and replay

**Context:** ROADMAP.md → Orchestrator DLQ

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## Component

API or orchestration

## Describe the feature

**Dead-letter path** for task orchestration after durable execution retry exhaustion. Operators inspect and **replay** failed orchestration runs.

## Use case

Transient AWS faults or code bugs can exhaust durable retries with no durable artifact for ops. Webhook processor has DLQ (#284); orchestrator does not.

## Proposed solution

1. Configure DLQ on durable orchestrator Lambda (SQS).
2. Payload: task_id, checkpoint, error, timestamp.
3. CloudWatch alarm on DLQ depth (#117 pattern).
4. Operator API/CLI: list DLQ messages, replay with idempotency guard.
5. Document runbook in `docs/design/ORCHESTRATOR.md`.

## Other information

- Distinct from FanOutConsumer DLQ (#117).
- Design context: `docs/design/ORCHESTRATOR.md`.

- [ ] This might be a breaking change

Start with ROADMAP.md and docs/design/ORCHESTRATOR.md, then compare the webhook processor DLQ in #284 and the CloudWatch alarm pattern in #117. Define the DLQ, payload, alarm, operator API/CLI listing and replay flow, idempotency guard, and runbook; done means these pieces are implemented and documented.

Written by the indexing model from the issue text.