Update README CI/CD badge to use GitHub Actions

- Replace CircleCI badge with GitHub Actions badge for main branch
- Point to the ci.yml workflow that replaced CircleCI
- Aligns with the CI/CD migration completed earlier
- Badge now shows status of our comprehensive GitHub Actions pipeline

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: dfcoffin

README.md

@@ -1,4 +1,4 @@
-[![CircleCI](https://circleci.com/gh/GreenButtonAlliance/OpenESPI-Common-java/tree/master.svg?style=svg)](https://circleci.com/gh/GreenButtonAlliance/OpenESPI-Common-java/tree/master)
+[![CI/CD Pipeline](https://github.com/GreenButtonAlliance/OpenESPI-Common-java/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/GreenButtonAlliance/OpenESPI-Common-java/actions/workflows/ci.yml)
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=GreenButtonAlliance_OpenESPI-Common-java&metric=alert_status)](https://sonarcloud.io/dashboard?id=GreenButtonAlliance_OpenESPI-Common-java)