Add AGENTS.md (#5996)

Author: Glavo

AGENTS.md

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+# Java Code Style Requirements
+
+These rules apply to all Java code written or modified in this repository.
+
+## Nullability
+
+- Annotate every class with JetBrains Annotations `@NotNullByDefault`.
+- Any type, field, parameter, return value, local variable, or generic type argument that may be `null` must be explicitly annotated with `@Nullable`.
+- Nullability must never be implicit.
+
+## Immutability
+
+- Immutable arrays and collections must be explicitly annotated with JetBrains Annotations `@Unmodifiable` or `@UnmodifiableView` as appropriate.
+- For arrays, use type-use syntax such as `String @Unmodifiable []`.
+
+## Documentation
+
+- Every class, field, and method must have documentation.
+- Documentation must use `///` Markdown-style Javadoc comments.
+- Keep documentation accurate and specific to the actual behavior, constraints, and side effects.
+- Add concise implementation comments inside complex logic whenever they materially improve readability or explain non-obvious behavior.
+
+## Gradle
+
+- When invoking Gradle in this repository, always set `GRADLE_USER_HOME` to the workspace-local `.gradle-user-home` directory.
+- Prefer commands such as `./gradlew -g .gradle-user-home ...` or the equivalent environment-variable-based configuration.
+- When running Gradle `test` tasks, use a higher timeout of ten minutes.