CIFSClient throws NullReferenceException on Linux when no cached auth-store entry exists for a UNC repository

[joaovent15]

Summary

On any non-Windows platform, constructing any BaseCommand-derived command crashes with an
unhandled NullReferenceException if the repository's repositories.json lists a UNC-style
repository and no credentials have ever been saved for it via cmf login. The crash happens during
CLI bootstrap/command-tree discovery — before any command's actual logic runs — so it breaks every
plugin command uniformly, not just the one the user is trying to run.

Where

• core/Objects/ExecutionContext.cs, line 115 — the CIFS-connecting block is gated by
  !RunningOnWindows && RepositoriesConfig.Repositories.HasAny(), so it's silently skipped on
  Windows and only exercised on Linux/macOS.

• core/Objects/CIFSClient.cs, constructor (lines 30-36) — the actual crash site:

  public CIFSClient(Uri uri, ISMBClient smbClient = null)
  {
      var authStore = ExecutionContext.ServiceProvider.GetService<IRepositoryAuthStore>();
      Server = uri.Host;
      _smbClient = smbClient ?? new SMB2Client();
      _credentials = authStore.GetCredentialsFor<CIFSRepositoryCredentials>(authStore.GetOrLoad().GetAwaiter().GetResult(), uri.AbsoluteUri);
      // ^ NullReferenceException here when authStore has no entry at all for this repository