chore: Maven Central publishing via Sonatype

[hermanngeorge15]

Summary

Set up Maven Central publishing so users can pull budget-breaker from Maven Central.

Scope

• Configure maven-publish plugin in build.gradle.kts
• POM metadata (description, license, SCM, developers)
• GPG signing configuration
• Sonatype OSSRH staging repository
• GitHub Actions workflow for automated publishing on tag push

Acceptance Criteria

• ./gradlew publishToMavenLocal produces correct POM and JAR
• GPG signing works with CI secrets
• GitHub Actions publishes to Sonatype staging on v* tag
• Artifact available on Maven Central as dev.unityinflow:budget-breaker
• Published POM includes correct metadata