dsh-spill-s3: S3-Compatible Spill Storage Backend for DeepSeek Harness

`dsh-spill-s3` is a plugin for DeepSeek Harness (DSH) that spills extremely long tool outputs to S3-compatible object storage (such as AWS S3, MinIO, and R2), replacing the default local disk storage. By implementing the `ctx.spillStore` interface, it addresses data persistence issues in containerized deployments. During installation, it automatically disables `spill-local`, while the storage bucket and credentials must be configured manually. The plugin supports multiple retrieval modes (such as CLI and presigned URLs) and uses native crypto for signing, with no AWS SDK dependency.