Add GitHub Actions CI workflow

[kevinwang1975]

Summary

• Adds a CI pipeline using GitHub Actions that builds and tests the project on Java 8 and Java 11 (matrix build)
• Triggers on push/PR to master, weekly schedule (Mondays 09:00 UTC), and manually via workflow_dispatch
• Caches Maven dependencies for faster subsequent runs
• Uploads the built JAR (target/pathfinder-1.0.jar) as a workflow artifact