Add Local Learning Mode to DeepSeek Harness with dsh-explain

### 标准技术译法版本（符合开源项目文档规范）： `dsh-explain` is a DSH learning mode plugin maintained by yuezengwu, licensed under MIT. It maintains a cross-session learning thread locally, retains at most one active explanation per source, adapts to the user's knowledge level via `ExplainContext`, and does not inject code into the main Agent. Currently compatible with DSH 0.1.0-rc.6 Web edition, users can trigger explanations, toggle features and run diagnostics via the `/explain` command, the selection shortcut menu, and the settings page.