build(ci): add NuGet config for private package feed

Add nuget.config with Gitea package source for MarketAlly.* packages and credential configuration using environment variables. Remove nuget.config from .gitignore to track package source configuration while keeping credentials secure through environment variable substitution.

.gitignore

@@ -54,7 +54,6 @@ coverage/
 # NuGet
 packages/
 *.nupkg
-nuget.config
 
 # Generated proto files
 **/Proto/

nuget.config

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="gitea" value="https://git.marketally.com/api/packages/marketally/nuget/index.json" />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+  <packageSourceMapping>
+    <packageSource key="gitea">
+      <package pattern="MarketAlly.*" />
+    </packageSource>
+    <packageSource key="nuget.org">
+      <package pattern="*" />
+    </packageSource>
+  </packageSourceMapping>
+  <packageSourceCredentials>
+    <gitea>
+      <add key="Username" value="%NUGET_USER%" />
+      <add key="ClearTextPassword" value="%NUGET_TOKEN%" />
+    </gitea>
+  </packageSourceCredentials>
+</configuration>