Create AGENTS.md

Author: mackysoft

AGENTS.md

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+# Unity-SerializeReferenceExtensions — AGENTS.md
+
+## Project overview
+This repository provides editor tooling for Unity's [SerializeReference], including `SubclassSelector`
+to pick concrete implementations of interfaces / abstract types in the Inspector.
+
+Primary risk area: type discovery & filtering (what types appear in the selector).
+
+## Repository layout
+- Repository URL: `https://github.com/mackysoft/Unity-SerializeReferenceExtensions`
+- Package source: `Assets/MackySoft/MackySoft.SerializeReferenceExtensions`
+- Editor code: `.../Editor/**`
+- Tests: `.../Tests/**`
+
+## Unity compatibility (critical)
+- Minimum supported Unity: 2021.3 (baseline for development/testing).
+- Unity 2023.2+ has enhanced generic type support (variance, etc.). Changes must not break 2021.3 behavior and guarded by `UNITY_2023_2_OR_NEWER`.
+
+## CI (GitHub Actions)
+I use GameCI `unity-test-runner`.
+- Always run EditMode tests.
+- Run a Unity matrix that includes:
+  - 2021.3.x (minimum baseline)
+  - 2023.2+ (generic/variance feature gate)
+
+## Architecture guardrails
+- Runtime surface area should remain minimal (mainly attributes / data structures).
+- Editor implementation (PropertyDrawer/UI/type search) must stay under `Editor/`.
+- Avoid introducing UnityEditor references into Runtime assemblies.
+
+## Coding conventions
+- Prefer `UnityEditor.TypeCache` for type discovery. Avoid full AppDomain scans unless necessary.
+- Keep allocations low in IMGUI paths (e.g., `OnGUI`).
+- Keep public API stable; if changing type filtering behavior, add/adjust tests.
+- As a general rule, you should follow the restrictions on SerializeReference in Unity's official documentation.
+  - https://docs.unity3d.com/ScriptReference/SerializeReference.html