chore: ignore Kotlin build caches outside the root module

The existing rule covered only /.kotlin/sessions/ at the repository root, so
build-logic/.kotlin/ showed up as untracked after any build.

.gitignore

@@ -70,4 +70,6 @@ fastlane/readme.md
 /app/release/output-metadata.json
 /app/release/
 /.kotlin/sessions/
+# Kotlin build caches in any module, not only the root - build-logic produces one too.
+.kotlin/
 .hermes/