dsh-searxng: Integrate SearXNG search for DeepSeek Harness

dsh-searxng is an MIT-licensed plugin (v0.2.1) for DSH designed to provide Agent-based Web search capabilities via SearXNG using `ctx.web`. The plugin supports two primary modes: automatically deploying a managed, loopback-only SearXNG Docker instance, or connecting to existing local or remote SearXNG instances without starting Docker. It requires Node.js 20+ and Docker Compose v2 (Docker path only) and does not support Podman. Core features include quickly initializing configuration and activating a Profile via the `setup` command, supporting port specification, and multi-profile environment isolation; it provides `