feat(web): ddgs plugin (second migration)

Adds plugins/web/ddgs/ following the same plugins/image_gen/ pattern as
brave_free. DuckDuckGo search via the community ddgs package; no API key,
package is an optional dep gated by is_available().

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

plugins/web/ddgs/plugin.yaml

@@ -0,0 +1,7 @@
+name: web-ddgs
+version: 1.0.0
+description: "DuckDuckGo web search via the ddgs Python package — no API key required. Install with `pip install ddgs`."
+author: NousResearch
+kind: backend
+provides_web_providers:
+  - ddgs