dsh-plugin-mgr: DeepSeek Harness Plugin Manager

`dsh-plugin-mgr` is the plugin management plugin for DeepSeek Harness (DSH), designed to manage plugins through a web-based card interface. Its core features include displaying plugins in card format, supporting enable/disable, uninstall, one-click updates (npm registry only), and search filtering; it uses a Host/Client architecture for configuration read/write and UI rendering. The plugin includes security restrictions (managing host modules is prohibited), concurrency protection, and immediate error display, effectively addressing the complexity of dependency management and status checking.