dsh-sandbox-mxc: MXC-Based DeepSeek Harness Sandbox Provider

`dsh-sandbox-mxc` is the sandbox provider implementation for DeepSeek Harness on the Windows platform. Built on Microsoft eXecution Containers (MXC), it wraps process parameters using `wxc-exec.exe` to achieve process isolation without kernel patches. The plugin depends on `@microsoft/mxc-sdk` to locate the executable, but avoids the SDK's Node.js version restrictions (requiring 24 or 26) by not importing native bindings, enabling it to run on Node.js 22+. After installation, it disables the default provider and supports configuration of file-system read/write permissions, network blocking, timeouts, and environment-variable injection, while providing fault tolerance by automatically filtering invalid paths.