Rubber Duck

### Describe the bug

rubber-duck reviews sometimes use the same model family as the primary session instead of selecting an independent reviewer, reducing the value of adversarial review.

i have observed this on a variety of models. most recently using 5.6 Terra - Max

### Affected version

v1.0.78

### Steps to reproduce the behavior

Have code/plan/contract whilst working in a gpt-5.6-terra session and request a rubber-duck review...

you then see this: Rubber-duck(gpt-5.6-terra) Assess audit design quality
└ Agent started in background with agent_id: audit-design-duck. You'll be notif...

### Expected behavior

I would expect a different model family selected from the current model being used in the session.

### Additional context

_No response_

Start by reproducing the rubber-duck review in the reported gpt-5.6-terra session and trace the model-selection entry point used for the background audit-design-duck agent. Done means the reviewer selects a different model family from the primary session while preserving the review request behavior.

Written by the indexing model from the issue text.