describe-changes@v1: with concatenate mode: append

✨ PR Description

Purpose: Add configurable reviewer option to bump-gitstream-core workflow to support deployment after GitHub SSO implementation.
Main changes:

• Added optional reviewer input parameter to workflow to replace hardcoded github.actor
• Created separate REVIEWER_ARG environment variable for PR creation command
• Modified gh pr create command to conditionally include reviewer based on input parameter

Generated by LinearB AI and added by gitStream.
_{AI-generated content may contain inaccuracies. Please verify before using. We'd love your feedback! 🚀}