feat(06-01): create GlobalSitesChangedMessage

- New ValueChangedMessage<IReadOnlyList<SiteInfo>> following TenantSwitchedMessage pattern
- Carries snapshot of globally selected sites (IReadOnlyList — immutable by design)

SharepointToolbox/Core/Messages/GlobalSitesChangedMessage.cs

@@ -0,0 +1,9 @@
+using CommunityToolkit.Mvvm.Messaging.Messages;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Core.Messages;
+
+public sealed class GlobalSitesChangedMessage : ValueChangedMessage<IReadOnlyList<SiteInfo>>
+{
+    public GlobalSitesChangedMessage(IReadOnlyList<SiteInfo> sites) : base(sites) { }
+}