dsh-ssh-plugin: SSH Execution and Persistent Connection Management

The DeepSeek Harness plugin `dsh-ssh-plugin` provides a native `ssh_exec` tool, enabling models to directly execute commands on remote Linux servers over SSH. Its core features include model tool integration, key-based persistent connection management, automatic recognition of local SSH configurations, and graceful degradation. The plugin supports both host-side and browser-side environments, simplifying agent operations on remote services, but requires the target environment to have bash installed and key-based authentication configured.