Add AGENTS guidelines

AGENTS.md

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+# 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.