dsh-search-router: A Native Web Search Routing Plugin for DeepSeek Harness

`dsh-search-router` is a lightweight plugin for DeepSeek Harness (DSH). It registers a native `WebSearchProvider` with `ctx.web`, supports multiple backends such as Exa, Tavily, Brave, and SearXNG, and includes a sequential fallback mechanism to ensure high availability for search calls. The plugin does not introduce new tools and provides both GUI drag-and-drop configuration and YAML file configuration. Installation is simple: it must be installed via CLI and requires the Web interface to be enabled. Notes include enabling JSON format for SearXNG, DuckDuckGo requiring no API key, and GUI settings overriding environment variables.