dsh-erhu-basic: Basic Erhu Learning Plan Generation Tool

`dsh-erhu-basic` is a DeepSeek Harness plugin maintained by user uckkk. It encapsulates the logic for generating basic erhu teaching plans as an `erhu_basic` tool that can be invoked directly in a session. The plugin is implemented purely in Node, has no network dependencies, requires `@deepseek-ai/cordis` and `@deepseek-ai/dsh-tools`, and is released under the MIT license.