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

[krokoko]

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

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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