New-CI: Implement Verify Signing Stage

[andrew-m-leonard]

Title: Create signature verification stage

Description:
Implement the Verify Signing stage that validates all signatures are correct and complete.

Tasks:

• Create 11-verify-signing.sh script
  • Read signed artifacts from INPUT_ARTIFACTS_DIR
  • Verify artifact signatures
  • Verify installer signatures
  • Verify GPG signatures
  • Write verification report to TARGET_DIR
• Add stage to Jenkinsfile.declarative
  • Define stage with proper prerequisites
  • Set INPUT_ARTIFACTS_DIR and TARGET_DIR
  • Add artifact archiving
• Add stage to run-pipeline.py
  • Implement stage_verify_signing() method
  • Set environment variables
  • Handle workspace cleanup
• Test stage locally
• Test stage in Jenkins
• Document stage behavior

Acceptance Criteria:

• Script works in both Jenkins and local
• All signature types verified
• Verification report generated
• Stage is restartable
• Documentation complete
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