dsh-agentenv-sandbox: AgentENV E2B Sandbox

The DeepSeek Harness plugin `dsh-agentenv-sandbox` uses an E2B adapter and the AgentENV Firecracker microVM backend to migrate an Agent's local filesystem and subprocess operations to a remote isolated environment, ensuring secure execution. It supports Bash, file tools, and LSP processes, and adopts a fail-closed policy to guarantee isolation without falling back to the host machine. Deployment requires a Node.js 22.19+ environment, a prebuilt AgentENV template, and specific environment variables. This solution addresses security risks in Agent development and enables secure, persistent sandboxed operations.