Force all interactions to use the selected model

Force all interactions to use the selected model

Add an option to globally enforce the user's selected model across all interactions, including named and unnamed sub-agents. This would ensure a consistent model experience and prevent background delegation to more expensive or unintended models.

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| Field | Value |
| --- | --- |
| App version | 1.1.8 |
| OS | Windows 10.0.26100 |
| Theme | Rainglow Codecourse |
| Path | /chat |
| Tenure | Week 9 |

Start from the /chat entry point and trace how the selected model is applied to interactions, including named and unnamed sub-agents. Done means an option can globally enforce the user's selected model and prevents unintended model delegation across those interactions.

索引モデルが issue の本文から書いたものです。