Subagent model setting allows primary-model overrides

[elkaix]

Problem

Selecting a subagent model in Settings configures only a default. The UI does not explain that an agent can explicitly request the primary or another model, so subagent tasks can run on the main model despite the selection.

Reproduction

1. Enable the experimental secondary-model setting.
2. Select a different subagent model.
3. Start a subagent with an explicit primary-model request.
4. Observe that the subagent uses the primary model.

Expected

Settings must describe the selection as a default and provide an option to pin all subagents to it through the canonical v2 force configuration.