DeepSeek Harness Plugin: dsh-attachment-s3

The `dsh-attachment-s3` plugin migrates attachment storage for DeepSeek Harness (DSH) from local to S3 cloud storage. It ensures data security through content addressing (SHA-256) and validation mechanisms, and supports AWS S3 as well as compatible services. The plugin replaces the built-in local backend via configuration, manages credentials using environment variables, and supports flexible storage prefixes and path styles. After installation, it requires configuration of the bucket name, region, and key references, making it suitable for scenarios that need cloud-based persistent storage or cross-device sharing of session context.