dsh-win-multi-bash: Windows Multi-Shell Environment Plugin

The DeepSeek Harness (DSH) plugin `dsh-win-multi-bash` introduces a `shell-select` executor and model tools to provide a unified interface for Windows agent development, supporting the routing of Git Bash, WSL, and PowerShell through `ctx.shell`. The plugin supports Shell auto-detection, path conversion, and multiple sandboxing mechanisms (such as `bwrap`). Developers can enhance security by configuring sandbox requirements and installing dependencies (such as `bubblewrap` in WSL). This plugin aims to resolve compatibility issues for multi-Shell invocations in Windows environments.