Update GitHub workflows to match atlas pattern

[ruhan1]

• Add explicit branch trigger (main) instead of wildcard push
• Add workflow_dispatch for manual triggering
• Switch from whelk-io/maven-settings-xml-action to s4u/maven-settings-action@v2.8.0
• Replace fork detection logic with event-based deployment (push vs PR)
• Update Sonatype server ID to central-portal-snapshots
• Add MAVEN_OPTS for better build performance
• Add watch trigger for starred repo builds
• Specify PR event types explicitly
• Make build and deployment steps clearer

This fixes the "Workflow runs completed with no jobs" issue by:

1. Making triggers more explicit
2. Simplifying conditional logic
3. Using more modern and reliable GitHub Actions