Write down the conventions for new code

CLAUDE.md: plain java.util collections instead of Guava's immutable ones,
javax.annotation.Nullable as the one nullability annotation, var for locals,
no Error Prone @immutable, and the Apache header without a copyright line on
a new file.

Author: abashev

CLAUDE.md

@@ -0,0 +1,35 @@
+# Working in this repository
+
+Building and testing: the README's "Building and testing locally" section. Gradle runs on the Temurin 21 that
+`mise install` provides, `./gradlew test` is what CI runs, and `./gradlew formatDiff` formats the files you changed.
+
+## Conventions for new code
+
+- **Collections.** Plain `java.util` types. `List.of`, `List.copyOf`, `Map.of` and `Stream.toList()` give
+  unmodifiable collections, so new code does not need Guava's `ImmutableList` or `ImmutableMap`; use them only
+  where an existing API already hands one over.
+- **Nullability.** One annotation, `javax.annotation.Nullable` (jsr305), the one the code uses; do not add another
+  `Nullable`.
+- **Local variables.** `var` wherever the initializer gives the type unchanged, in `for` loops too. Not where it
+  would box a primitive (`for (var n : List<Integer>)`), and not without an initializer that has a type of its own
+  (a lambda, `null`).
+- **Immutability annotations.** Do not add Error Prone's `@Immutable` or `@SuppressWarnings("Immutable")`.
+- **License header.** A new file starts with the Apache 2.0 header alone, without a copyright line:
+
+  ```java
+  /*
+   * Licensed under the Apache License, Version 2.0 (the "License");
+   * you may not use this file except in compliance with the License.
+   * You may obtain a copy of the License at
+   *
+   *     http://www.apache.org/licenses/LICENSE-2.0
+   *
+   * Unless required by applicable law or agreed to in writing, software
+   * distributed under the License is distributed on an "AS IS" BASIS,
+   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   * See the License for the specific language governing permissions and
+   * limitations under the License.
+   */
+  ```
+
+  Files inherited from google-java-format and palantir-java-format keep their original headers.