Changelog for release 0.13.1 (#1449)

Also update `AGENTS.md` to help coding agents auto-format release note
bullets properly.

Author: msridhar

AGENTS.md

@@ -8,3 +8,11 @@ test class or method within that module, you can use the `--tests` flag. For exa
 ```bash
 ./gradlew :nullaway:test --tests "com.uber.nullaway.NullAwayTest"
 ```
+
+# Changelog
+
+Our `CHANGELOG.md` file should be formatted as follows:
+
+* Link to PRs just via their number, e.g. `#1234`, not a full GitHub URL
+* Don't credit `@msridhar` in changelog entries, but credit all other contributors
+* Under maintenance, list sub-bullets with a `-` rather than a `*`

CHANGELOG.md

@@ -1,6 +1,15 @@
 Changelog
 =========
 
+Version 0.13.1
+--------------
+
+* Improve verification of !null -> !null contracts (#1441)
+* Substitute inferred `@NonNull` types for generic method inference (#1445)
+* Better support for some contracts with boolean argument constraints (#1447)
+* Maintenance
+  - Add junit-framework as another integration test (#1446)
+
 Version 0.13.0
 --------------