dsh-plugin-midscene: Adding Midscene-driven UI automation for DeepSeek Harness

dsh-plugin-midscene is an MIT-licensed plugin developed for DeepSeek Harness (DSH), designed to implement AI-driven UI automation through Midscene technology. The plugin provides two tools, `android_ui` and `web_ui`, through a single `ctx.midscene` interface, supporting models to locate and operate real Android devices or Chrome pages based on natural language. Core functions include operating on ADB-connected Android devices and Chrome pages with running debug ports. The tools support actions such as tap, input, query, and assert, and the visual model requires configuration via environment variables. Installation requires using DSH CLI.