websearch-dsh: Native Multi-Source Web Search Plugin for DeepSeek Harness

`websearch-dsh` is a DeepSeek Harness (DSH) plugin designed to replace the single `web_search` tool with a multi-provider fallback chain, addressing issues such as single-service outages or rate limits. It supports keyless DuckDuckGo search and allows multiple API keys (e.g., Tavily, Brave, etc.) to be configured via GUI toggles or environment variables. The plugin is easy to install, takes effect without a restart, and provides fault tolerance to ensure the stability of search services.