Make the setup-wizard multi-instance-aware

[JSONbored]

Problem: The GitHub App setup wizard is a one-time, one-operator flow today — it has no concept of provisioning a second, independent instance.

Area: ORB / Fleet ops

Proposal: Extend the wizard to support provisioning additional instances without conflicting with an already-configured one on the same infrastructure.

Deliverables:

• A wizard flow that supports more than one instance's worth of App credentials.

[linear-code]

JSO-3557