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

skills/avalonia-zafiro-development/SKILL.md

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+---
+name: avalonia-zafiro-development
+description: Mandatory skills, conventions, and behavioral rules for Avalonia UI development using the Zafiro toolkit.
+---
+
+# Avalonia Zafiro Development
+
+This skill defines the mandatory conventions and behavioral rules for developing cross-platform applications with Avalonia UI and the Zafiro toolkit. These rules prioritize maintainability, correctness, and a functional-reactive approach.
+
+## Core Pillars
+
+1.  **Functional-Reactive MVVM**: Pure MVVM logic using DynamicData and ReactiveUI.
+2.  **Safety & Predictability**: Explicit error handling with `Result` types and avoidance of exceptions for flow control.
+3.  **Cross-Platform Excellence**: Strictly Avalonia-independent ViewModels and composition-over-inheritance.
+4.  **Zafiro First**: Leverage existing Zafiro abstractions and helpers to avoid redundancy.
+
+## Guides
+
+- [Core Technical Skills & Architecture](core-technical-skills.md): Fundamental skills and architectural principles.
+- [Naming & Coding Standards](naming-standards.md): Rules for naming, fields, and error handling.
+- [Avalonia, Zafiro & Reactive Rules](avalonia-reactive-rules.md): Specific guidelines for UI, Zafiro integration, and DynamicData pipelines.
+- [Zafiro Shortcuts](zafiro-shortcuts.md): Concise mappings for common Rx/Zafiro operations.
+- [Common Patterns](patterns.md): Advanced patterns like `RefreshableCollection` and Validation.
+
+## Procedure Before Writing Code
+
+1.  **Search First**: Search the codebase for similar implementations or existing Zafiro helpers.
+2.  **Reusable Extensions**: If a helper is missing, propose a new reusable extension method instead of inlining complex logic.
+3.  **Reactive Pipelines**: Ensure DynamicData operators are used instead of plain Rx where applicable.