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

skills/avalonia-zafiro-development/patterns.md

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+# Common Patterns in Angor/Zafiro
+
+## Refreshable Collections
+
+The `RefreshableCollection` pattern is used to manage lists that can be refreshed via a command, maintaining an internal `SourceCache`/`SourceList` and exposing a `ReadOnlyObservableCollection`.
+
+### Implementation
+
+```csharp
+var refresher = RefreshableCollection.Create(
+        () => GetDataTask(), 
+        model => model.Id)
+    .DisposeWith(disposable);
+
+LoadData = refresher.Refresh;
+Items = refresher.Items;
+```
+
+### Benefits
+- **Automatic Loading**: Handles the command execution and results.
+- **Efficient Updates**: Uses `EditDiff` internally to update items without clearing the list.
+- **UI Friendly**: Exposes `Items` as a `ReadOnlyObservableCollection` suitable for binding.
+
+## Mandatory Validation Pattern
+
+When validating dynamic collections, always use the Zafiro validation extension:
+
+```csharp
+this.ValidationRule(
+        StagesSource
+            .Connect()
+            .FilterOnObservable(stage => stage.IsValid)
+            .IsEmpty(),
+        b => !b,
+        _ => "Stages are not valid")
+    .DisposeWith(Disposables);
+```
+
+## Error Handling Pipeline
+
+Instead of manual `Subscribe`, use `HandleErrorsWith` to pipe errors directly to the user:
+
+```csharp
+LoadProjects.HandleErrorsWith(uiServices.NotificationService, "Could not load projects");
+```