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.

No source files, tests, or implementation entry points are named. Start by reviewing the GitHub Enterprise Importer GitLab adapter and the `gh gl2gh` CLI extension, then check the preflight and remediation guidance; done means teams can complete a self-service GitLab-to-GitHub migration without required services engagement.

索引モデルが issue の本文から書いたものです。