Add contribution guidelines

Author: twisti-dev

AGENTS.md

@@ -0,0 +1,6 @@
+# Coding Guidelines for surf-cloud
+
+- Use Kotlin (with some Java) standard style: 4-space indentation, no trailing whitespace, end files with a newline.
+- Keep commit messages short (50 characters or less) and in the imperative mood.
+- After modifying any code, run `./gradlew build` in the repository root. The build must succeed before committing.
+- Summarise changes in the PR body and mention build results.