dsh-web-search-multi: DeepSeek Harness Multi-source Web Search Plugin

`dsh-web-search-multi` is a multi-provider Web search plugin for DeepSeek Harness (DSH), designed to unify access, configuration, and fallback logic for different search engines. The plugin supports keyless direct connection engines such as 360, Bing, and Baidu, as well as providers including Tavily, Brave Search, Serper/Google, Bocha AI, and self-hosted SearXNG, and includes a built-in DuckDuckGo global fallback mechanism. It provides a visual "Multi-source Web Search" settings page via the DSH Web UI, supporting real-time connection testing and instant persistence. Credentials are securely stored in `~/.dsh/.credentials.yaml`, and general settings are saved to `~/.dsh/.d`.