Add Gradle wrapper and CI workflow

- Commit gradlew, gradlew.bat and gradle/wrapper so ./gradlew build works
  from a fresh clone as the README instructs (Gradle 8.7)
- Fix .gitignore rule order: *.jar came after the wrapper-jar negation and
  the last matching rule wins, so the wrapper jar was still ignored
- CI on push and pull request: build (grammar with -Werror), test, and
  parseExamples

.gitignore

@@ -125,6 +125,8 @@ local.properties
 CLAUDE.md
 
 # Gradle wrapper (DO NOT IGNORE)
+# The negation must come after *.jar above — the last matching rule wins.
 !gradle/
+!gradle/wrapper/gradle-wrapper.jar
 !gradlew
 !gradlew.bat