New-CI: Implement GPG Sign Stage

[andrew-m-leonard]

Title: Create GPG signing stage for artifacts

Description:
Implement the GPG Sign stage that creates GPG signatures for all artifacts.

Tasks:

• Create 10-gpg-sign.sh script
  • Read artifacts from INPUT_ARTIFACTS_DIR
  • Create GPG signatures (.asc files)
  • Write signatures to TARGET_DIR
  • Generate signing report
• Add stage to Jenkinsfile.declarative
  • Define stage with proper prerequisites
  • Set INPUT_ARTIFACTS_DIR and TARGET_DIR
  • Add artifact archiving
  • Handle GPG keys securely
• Add stage to run-pipeline.py
  • Implement stage_gpg_sign() method
  • Set environment variables
  • Handle workspace cleanup
• Test stage locally (with test keys)
• Test stage in Jenkins
• Document stage behavior

Acceptance Criteria:

• Script works in both Jenkins and local
• GPG signatures created correctly
• Keys handled securely
• Stage is restartable
• Documentation complete
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