DeepSeek Harness Plugin: dsh-group-buy

The `dsh-group-buy` plugin for DeepSeek Harness (DSH) provides group-buy calculation capabilities and is maintained by uckkk. Implemented in pure Node.js with no network dependencies, it can return detailed calculation results based on input parameters via the `group_buy` tool. Developers can install and enable the plugin with a command, making it suitable for local logic scenarios that require group-buy calculations.