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/plugin.yaml

@@ -0,0 +1,7 @@
+name: web-searxng
+version: 1.0.0
+description: "SearXNG web search — free, self-hosted, privacy-respecting metasearch engine. Requires SEARXNG_URL pointing at your instance."
+author: NousResearch
+kind: backend
+provides_web_providers:
+  - searxng