dsh-local-plugin-installer: Local Plugin Installer

`dsh-local-plugin-installer` is a web plugin for DeepSeek Harness (DSH) designed to unify the local plugin development workflow. It adds a "Settings → Plugins → Install" page to DSH Web, allowing developers to install local directories or Git sources directly in the browser and run `pnpm build` to build and debug without switching to a terminal. The plugin sends commands to the DSH process via the Web UI, and the Web service must be restarted after installation for the changes to take effect. When using the plugin, choose only trusted plugin directories, as the build process may execute arbitrary code.