dsh-windows-shell-policy: Windows Default Shell Policy Plugin for DeepSeek Harness

`dsh-windows-shell-policy` is the default Shell policy plugin for DeepSeek Harness (DSH), designed to resolve the tool interface confusion caused by the coexistence of Bash and PowerShell on Windows. By detecting local Git-Bash, MSYS2, or Cygwin, the plugin provides a "Default Shell" configuration card in settings, supporting three modes: Automatic, Bash, and PowerShell. Upon activation, if Bash is selected, the plugin dynamically registers the Bash tool and trims prompts to ensure the Agent interacts with a single Shell tool, thereby simplifying interaction logic. The configuration supports persistence and is retained after restarts; uninstallation fully restores tool registration and prompt states. Install...