dsh-web-search-local: Keyless Multi-Engine Web Search Plugin

This plugin registers a local provider to resolve DeepSeek Harness's default reliance on DeepSeek server-side search, enabling local models to directly invoke web search and page fetching. It supports multi-engine web search and page fetching with a layered strategy (SearXNG -> Global -> CN), proxy configuration, charset awareness, and resilience mechanisms for rate limiting. After installation, it automatically switches the provider; Node.js >= 20.3 is required.