Feature flag rollout: auto_merge_on_merge_status_change

Summary

This issue tracks the rollout of the auto_merge_on_merge_status_change feature flag introduced in !217382.

Feature Description

This feature adds a trigger for AutoMergeProcessWorker when a merge request's merge status changes to can_be_merged and auto merge is enabled.

Feature Flag

• Name: auto_merge_on_merge_status_change
• Type: gitlab_com_derisk
• Introduced in: !217382

Rollout Plan

Stage 1: GitLab.com - Small percentage

• Enable for 5% of projects
• Monitor for errors in Sentry
• Verify auto merge triggers correctly

Stage 2: GitLab.com - Wider rollout

• Increase to 25%, then 50% of projects
• Continue monitoring

Stage 3: GitLab.com - Full rollout

• Enable for 100% of GitLab.com
• Monitor for 1-2 weeks

Stage 4: Cleanup

• Remove feature flag from code
• Create cleanup MR

Rollback Plan

Disable the feature flag: /chatops run feature set auto_merge_on_merge_status_change false