dsh-web-search-searxng: Self-hosted SearXNG Provider for DSH Web Search

`dsh-web-search-searxng` is a community-maintained DSH plugin that provides search capabilities to `ctx.web` through the `/search?format=json` endpoint of a self-hosted SearXNG instance. Unlike calling the full API, this plugin requires no API key and consumes no model turns per search, significantly reducing operational costs. It does not register new tools but reuses the existing `web_search` tool from `@deepseek-ai/dsh-tool-web`, mapping SearXNG results to standard sources and contents with deduplication. The plugin supports various optional configurations such as language, engines, timeout, and snippet length, with changes taking effect immediately. Installation requires adding it from npm or GitHub, and setting the