Org migration target cell: Core Rails behavior

What does this MR do and why?

This implements the core Rails behavior of an org migration target cell, built on top of the foundation Org migration target cell: Add foundation (!223454 - merged)+.

Part of Org Mover: Implement a full E2E Org Move test i... (#546267)

References

MR	Status
Org migration target cell: Add foundation (!223454 - merged)	✅
Org migration target cell: Core Rails behavior (!225219)	You are here
Draft: Implement behavior of org migration targ... (!219850)

How to set up and validate locally

See gitlab-development-kit!5555

Run the end-to-end script which installs GDK from scratch and actually uses the final Rails MR Draft: Implement behavior of org migration targ... (!219850):

# Set your license (required - Premium or Ultimate)
export GITLAB_ACTIVATION_CODE="your-activation-code"
# OR
export GITLAB_LICENSE_FILE="/path/to/license.gitlab-license"

# Optional: customize branches (these are the defaults)
export GDK_BRANCH="mk/gdk-org-configure-replication"
export GITLAB_BRANCH="mk/org-migration-target"

# Run from an empty directory
mkdir org-migration-test && cd org-migration-test

# Download and run the script
curl -fsSL "https://gitlab.com/gitlab-org/gitlab-development-kit/-/raw/mk/gdk-org-configure-replication/support/cells/org-migration-install" -o org-migration-install
chmod +x org-migration-install
./org-migration-install

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.