Add AGENTS.md

[Copilot]

Documents the repository for AI agents and contributors, with primary focus on the two actively developed libraries: Mvvm.Controls and Mvvm.Controls.SourceGenerators.

Coverage

• Build & test — exact commands, EnableWindowsTargeting=true requirement on non-Windows, TFM matrix, CI overview
• Project map — which libraries are active vs. complete
• Mvvm.Controls — namespace/directory layout, control and command class hierarchies
• Source generator architecture — three Roslyn-version builds (3.11 / 4.10 / 5.0), Parse → Data → Emit pattern, DataOrDiagnostics<T>, diagnostic ID reference table (RFB_MVVM_000–013)
• Consumer attribute API — what each attribute ([Button], [Selector], [ComboBox], [ListBox], etc.) generates and the property-naming convention
• Testing conventions — MSTest, GeneratorInputs/ embedded resources, what to update when adding/modifying a generator
• Key conventions — nullability, partial requirement, async method signatures, XAML namespace URIs

[Copilot]

Pull request overview

Adds an AGENTS.md guide to help AI agents and contributors navigate and work in the repository, focusing on the active Mvvm.Controls and source generator work.

Changes:

• Adds build/test/pack and CI workflow guidance (including EnableWindowsTargeting=true context and TFM matrix).
• Documents Mvvm.Controls structure, key primitives/command hierarchies, and XAML namespace URIs.
• Documents source generator architecture (multi-Roslyn builds, Parse → Data → Emit, diagnostics/attributes overview) and testing conventions.

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