Rollout: resolve_merge_conflicts_flow feature flag
## Summary
Feature flag rollout issue for `resolve_merge_conflicts_flow`, which gates the Resolve Merge Conflicts foundational flow.
Related to #588529 and &20688.
## Rollout plan
- [ ] Enable on staging
- [ ] Enable on GitLab.com for internal projects
- [ ] Enable percentage rollout (10% → 25% → 50% → 100%)
- [ ] Verify metrics and error rates at each stage
- [ ] Remove feature flag and clean up
issue