feat(web): searxng plugin (search-only, third migration)

Adds plugins/web/searxng/. SearXNG aggregates results from upstream engines
via its JSON API (/search?format=json) — search-only, no extract capability
(supports_extract() returns False).

E2E verified — registry now has ['brave-free', 'ddgs', 'searxng'].

plugins/web/searxng/__init__.py

@@ -0,0 +1,15 @@
+"""SearXNG search plugin — bundled, auto-loaded.
+
+Backed by a user-hosted SearXNG instance (URL configured via ``SEARXNG_URL``).
+Search-only — pair with an extract provider (firecrawl/tavily/exa) for
+``web_extract`` calls.
+"""
+
+from __future__ import annotations
+
+from plugins.web.searxng.provider import SearXNGWebSearchProvider
+
+
+def register(ctx) -> None:
+    """Register the SearXNG provider with the plugin context."""
+    ctx.register_web_search_provider(SearXNGWebSearchProvider())