dsh-speech-plugin: Speech plugin for DeepSeek Harness

dsh-speech-plugin is an independent voice plugin for DeepSeek Harness that adds voice playback and input capabilities to DSH via a plugin mechanism without the need to modify source code. Its main features include single-message playback, session-level automatic playback (effective only for new replies), and voice input via microphone. Playback and input can utilize Alibaba Bailian and Volcengine Doubao engines separately or simultaneously. If the cloud service fails, playback falls back to the browser's system voice, while voice input is disabled and the reason is prompted. To install, execute the `dsh plugin add dsh-speech-plugin` command, configure the `SPEECH_DASHSCOPE_API_KEY` or `SPEECH_VOLCENGINE_API_KEY` environment variables (avoid starting with `DSH_`), and finally restart.