feat: add gpt secondary monthly review

[Pigbibi]

Summary

• add GPT secondary review after the existing Claude monthly review workflow
• persist issue context, primary markdown, secondary JSON, and final combined markdown as workflow artifacts
• keep posting a single issue comment while switching it to the combined dual-AI review output

Testing

• python3 -m unittest tests/test_post_monthly_ai_review_comment.py tests/test_render_monthly_ai_review.py tests/test_run_openai_secondary_review.py tests/test_monthly_publish_workflow_config.py
• uvx ruff check scripts tests
• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].each { |f| YAML.load_file(f) }; puts "yaml ok"'