dsh-session-s3: S3-backed SessionPersistence Plugin for DeepSeek Harness

`dsh-session-s3` is an MIT-licensed community plugin designed to provide S3-based session persistence for DeepSeek Harness. It adopts a `wal3-Lite` design, using immutable JSONL fragments and CAS manifests with `If-None-Match` to coordinate concurrent writes and ensure data consistency. The plugin supports the AWS SDK credential chain as well as S3-compatible endpoints such as R2 and MinIO. Users must install it to a specific profile (e.g., `web`) via the `dsh plugin` command rather than globally, and the runtime environment requires Node.js 22 or higher. During configuration, users must specify the required `bucket` in `cordis.patch.yml`.