Show the model used by background agents

Background agent results currently don’t make it clear which model produced them. This is especially important for review agents such as `rubber-duck`, where knowing the model helps users interpret the feedback.

Expose the model used for each background agent in the Insights tab, the chat transcript, or both.

**Acceptance criteria**

- Users can identify the exact model used by a background agent.
- Model information remains visible after the agent completes.
- The model is shown for `rubber-duck` reviews and other background agent types.
- The display handles default and explicitly overridden models.