Skip base-realm instances when expanding listing examples for submission

[richardhjtan]

Summary

• Catalog submissions threw Cannot install instance from base realm
  whenever an example instance had a relationship into the base realm
  (e.g. a cardInfo.theme pointing at a shared Theme on base).
• expandInstances in collect-submission-files.ts was BFS-walking
  relationships and queuing every related id, including base-realm
  ones, which then hit the guard in planInstanceInstall.
• Now we skip base-realm ids in both the seed queue and the
  relationship walk. Base references stay as absolute URLs in the
  produced JSON (base is shared infra and always exists).

Test plan

• Submit a listing whose example has a relationship to a base-realm
  card (e.g. FitnessGoal with cardInfo.theme →
  https://cardstack.com/base/Theme/cardstack-brand-guide); verify
  the submission completes and the absolute base URL is preserved
  in the example JSON.