feat: add Avalonia Zafiro development, layout, and viewmodel skills

skills/avalonia-viewmodels-zafiro/viewmodels.md

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+# ViewModels & Commands
+
+In a Zafiro-based application, ViewModels should be functional, reactive, and resilient.
+
+## Reactive ViewModels
+
+Use `ReactiveObject` as the base class. Properties should be defined using the `[Reactive]` attribute (from ReactiveUI.SourceGenerators) for brevity.
+
+```csharp
+public partial class MyViewModel : ReactiveObject
+{
+    [Reactive] private string name;
+    [Reactive] private bool isBusy;
+}
+```
+
+### Observation and Transformation
+
+Use `WhenAnyValue` to react to property changes:
+
+```csharp
+this.WhenAnyValue(x => x.Name)
+    .Select(name => !string.IsNullOrEmpty(name))
+    .ToPropertyEx(this, x => x.CanSubmit);
+```
+
+## Enhanced Commands
+
+Zafiro uses `IEnhancedCommand`, which extends `ICommand` and `IReactiveCommand` with additional metadata like `Name` and `Text`.
+
+### Creating a Command
+
+Use `ReactiveCommand.Create` or `ReactiveCommand.CreateFromTask` and then `Enhance()` it.
+
+```csharp
+public IEnhancedCommand Submit { get; }
+
+public MyViewModel()
+{
+    Submit = ReactiveCommand.CreateFromTask(OnSubmit, canSubmit)
+        .Enhance(text: "Submit Data", name: "SubmitCommand");
+}
+```
+
+### Error Handling
+
+Use `HandleErrorsWith` to automatically channel command errors to the `NotificationService`.
+
+```csharp
+Submit.HandleErrorsWith(uiServices.NotificationService, "Submission Failed")
+    .DisposeWith(disposable);
+```
+
+## Disposables
+
+Always use a `CompositeDisposable` to manage subscriptions and command lifetimes.
+
+```csharp
+public class MyViewModel : ReactiveObject, IDisposable
+{
+    private readonly CompositeDisposable disposables = new();
+
+    public void Dispose() => disposables.Dispose();
+}
+```
+
+> [!TIP]
+> Use `.DisposeWith(disposables)` on any observable subscription or command to ensure proper cleanup.