syy-shark/dsh-music-plugin: Add a local music switch for DeepSeek Harness

dsh-music-plugin is a local music control plugin for DeepSeek Harness (DSH), version 0.1.0, released under the MIT License. It enables Agents to directly manipulate audio files in local folders, supporting search, playback (play, pause, resume), track skipping, and status queries. Core features include audio control via tool calls such as `music_search`, queue playback support, prioritizing mpv or afplay backends, and allowing other DSH plugins to reuse its capabilities through `ctx.music`. Usage requires placing songs in a local directory (default ~/Music), installing the plugin via the `npx` command (specifying a local path), and starting DSH Web to view the playlist and perform basic controls on the web interface, or directly...