fix: stop review PR flow before merge

[haosenwang1018]

Summary

Fix the --pr workflow so it creates a review PR and stops, while keeping --ship as the mode that continues into merge/auto-complete.

Related Issues

Fixes #452

Changes Made

• persist ship intent as autoMerge in cluster start options
• persist/pass autoMerge through orchestrator PR options into git-pusher generation
• update git-pusher-template prompt generation so review-only PR mode stops after PR creation instead of always forcing step 6 merge
• only require merge_commit_sha in structured output when merge/auto-complete is actually expected
• add regression tests covering review-only vs ship mode schema/prompt behavior

Testing

• node -c src/agents/git-pusher-template.js
• direct generateGitPusherAgent() checks for review-only and ship mode behavior
• npx mocha tests/structuredOutput-mapping.test.js

Checklist

• Tests pass (npm test)
• Documentation updated (if needed)
• Follows commit guidelines