fix(mirror): point NuGet source to nuget-group (was nuget-proxy which doesn't exist)

nuget-group aggregates nuget.org-proxy (Liara) and nuget-runflare-proxy
(Runflare) — automatic fallback if one upstream is down.

npm-proxy and docker-hub-proxy names match exactly, no changes needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nuget.mirror.config

@@ -6,7 +6,9 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="nexus-nuget" value="http://mirror:8081/repository/nuget-proxy/index.json" protocolVersion="3" />
+    <!-- nuget-group aggregates nuget.org-proxy (Liara) + nuget-runflare-proxy (Runflare).
+         If Liara is down, Nexus automatically falls back to Runflare. -->
+    <add key="nexus-nuget" value="http://mirror:8081/repository/nuget-group/index.json" protocolVersion="3" />
   </packageSources>
   <config>
     <add key="http_retry_count" value="8" />