New-CI: Implement Sign Installers Stage

[andrew-m-leonard]

Title: Create installer signing stage

Description:
Implement the Sign Installers stage that signs installer packages with appropriate certificates.

Tasks:

• Create 09-sign-installers.sh script
  • Read installers from INPUT_ARTIFACTS_DIR
  • Sign with platform-specific certificates
  • Write signed installers 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 signing credentials securely
• Add stage to run-pipeline.py
  • Implement stage_sign_installers() method
  • Set environment variables
  • Handle workspace cleanup
• Test stage locally (with test certs)
• Test stage in Jenkins
• Document stage behavior

Acceptance Criteria:

• Script works in both Jenkins and local
• Installers signed correctly
• Platform-specific signing handled
• Stage is restartable
• Documentation complete