Add AGENTS instructions (#21)

Author: LucHeart

AGENTS.md

@@ -0,0 +1,17 @@
+# AGENTS Instructions
+
+## Build requirements
+- This project targets **.NET 9**. Install the .NET 9 SDK before running any `dotnet` commands.
+- When working on Linux, use the `Release-Photino` or `Release-Web` configurations. The Windows build configurations (`*Windows`) require MAUI and only work on Windows.
+
+## Running checks
+- There are currently no unit tests. To verify the project builds, run:
+  ```bash
+  dotnet publish Desktop/Desktop.csproj -c Release-Photino -o ./publish/Photino-Linux
+  ```
+  or for the web version:
+  ```bash
+  dotnet publish Desktop/Desktop.csproj -c Release-Web -o ./publish/Web-Linux
+  ```
+- Ensure these commands succeed before committing changes.
+