Migrate GitLab Repositories to GitHub with GitHub Enterprise Importer (GEI) [GA]

### Value Prop
You can now migrate your GitLab repositories to GitHub entirely on your own — no Expert Services engagement or partner agreement required. GitHub Enterprise Importer includes a production-ready GitLab adapter, the new `gh gl2gh` CLI extension, automated preflight checks, and clear remediation guidance so you can run migrations confidently on your own schedule. The experience is on par with all other supported migration paths in GitHub Enterprise Importer.

### Expected Outcome
Teams moving from GitLab to GitHub should be able to start and complete their migration without procurement delays, scheduling friction, or added cost from required services engagements. By making this path fully self-serve and generally available, more organizations can act on their own timelines while getting predictable, high-quality migration outcomes from end to end.

未指定源文件、测试或实现入口。首先查看 GitHub Enterprise Importer 的 GitLab 适配器和 `gh gl2gh` CLI 扩展，然后检查 preflight 和 remediation 指南；当团队无需服务团队介入即可完成自助式 GitLab 到 GitHub 迁移时，即视为完成。

由索引模型根据 Issue 内容生成。