feat(agent): S3-backed Claude SDK session store

**Context:** ROADMAP.md → S3-backed SDK session store (portable transcripts)

---

## Component

Agent (Python runtime)

## Describe the feature

Plumb the Claude Agent SDK **`SessionStore`** to S3 (dedicated bucket or prefix) with:

- Eager flush, IAM-scoped access, conditional part creates, checksums, adaptive retries, structured logging
- Metrics/alarms on transcript mirror failures
- Graceful shutdown (`disconnect` on SIGTERM/cancel) so in-flight frames flush
- Persist **`task_id` ↔ Claude session UUID** (from first `ResultMessage`) for resume on another worker
- Keep agent **`cwd` stable** so SDK-derived `project_key` paths stay predictable
- Plan **compaction** when part count threatens resume latency
- Optional **S3 Express One Zone** when fleet is single-AZ

Complements shipped **Persistent session storage** (`/mnt/workspace` FUSE caches) and end-of-task **trace** upload to `traces/...jsonl.gz`.

## Use case

In-VM caches do not survive worker migration or long pauses. Portable transcripts enable resume, debugging, and compliance without relying solely on end-of-task trace upload.

## Proposed solution

1. S3 prefix per `task_id` + SDK `project_key`; SessionRole scoped writes.
2. Flush on frame boundary; `disconnect` on shutdown.
3. Alarms on mirror failures; structured logging.
4. Compaction when part count threatens resume latency.
5. Optional S3 Express One Zone when fleet is single-AZ.
6. Map first `ResultMessage` session UUID to `task_id` in DynamoDB for cross-worker resume.

## Other information

- Shipped: persistent session storage on `/mnt/workspace`; execution tracing to `traces/...jsonl.gz`.
- Design context: `docs/design/COMPUTE.md`, `docs/design/OBSERVABILITY.md`.

- [ ] This might be a breaking change

Bắt đầu với ROADMAP.md và ngữ cảnh thiết kế trong docs/design/COMPUTE.md và docs/design/OBSERVABILITY.md. Sau đó, xác định các entry point tải lên đã được cung cấp cho /mnt/workspace session storage và traces/...jsonl.gz. Được xem là hoàn tất khi một triển khai đã thống nhất bao quát việc mirror SessionStore portable, ánh xạ session giữa các worker, flush khi shutdown, cảnh báo lỗi, retry và compact.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.