Respect Pi’s scoped models in the provider model picker

[wy3z]

Summary

Make BB’s Pi model picker respect Pi’s workspace-aware scoped model configuration and selected default.

Pi now runs through the provider-pi plugin and exposes its model catalog over RPC. The catalog previously returned every authenticated model, ignoring the scope and default Pi applies to its own picker.

Changes

• Report Pi’s resolved scoped models and selected default through the existing private extension channel.
• Synchronize model-scope state before serving a catalog result.
• Preserve Pi’s scope ordering in BB’s model picker.
• Use Pi’s selected model as the picker default when available.
• Keep explicitly scoped dated model snapshots selectable.
• Isolate catalog scope state by child-process generation so a restarted Pi child cannot inherit stale scope or default state.
• Fall back to the full available catalog when Pi reports no active scope.

Testing

• Pi provider suite: 20 files, 116 tests passed.
• Pi provider Turbo typecheck passed.
• git diff --check passed.
• Added a process-backed regression test that:
  • starts a scoped catalog child,
  • forces it to exit,
  • changes the scope for the replacement child,
  • verifies the new generation’s catalog and scope are used together.
• Tested against a real local Pi installation and workspace configuration:
  • only the five configured scoped models were returned,
  • scope ordering was preserved,
  • the configured first model was selected as default,
  • repeated catalog requests returned the same scoped result.