chore: add NuGet.Config to pin restore to nuget.org only

[artcava]

Summary

Adds a repo-level NuGet.Config that clears all inherited package sources and
declares only nuget.org as the authoritative feed.

Problem

The global NuGet configuration on the development machine includes other
feed from an unrelated project. Every dotnet restore in this repo contacted
that endpoint — generating NU1900 warnings and slowing down restores on
machines without access to that Azure DevOps organisation.

Change

NuGet.Config at repo root:

<packageSources>
  <clear />
  <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
</packageSources>

[github-actions]

⚠️ This PR does not reference an approved issue. Please link to an issue using "Closes #123" in the description.