Honor explicit startup model selection like /model after initialization #5213

Describe the feature or problem you would like to solve

At startup, Copilot CLI reports that an administrator-managed setting replaced the preferred model with auto. Selecting a model manually with /model afterward works, but repeating that step in every session is inconvenient.

The CLI already provides --model <model>. Please make explicit startup selection behave consistently with a permitted manual /model <model> selection after initialization, or provide a supported post-startup model-selection option.

Proposed solution

Apply an explicitly requested session model once startup configuration is resolved, with the same availability and policy validation as /model. If administrator policy genuinely prohibits that model, reject the selection with a clear explanation. This request is not to bypass enforced model restrictions; it is to automate a selection that the interactive CLI already permits.

Please document precedence among administrator-managed defaults, user defaults, --model, and interactive /model selection.

Example prompts or workflows

• Launch copilot --model <permitted-model> and use that model without an additional interactive command.
• Use a shell alias or wrapper for a preferred permitted model across sessions.
• Resume with copilot --continue --model <permitted-model> without manually reselecting the model.

Additional context

Observed with Copilot CLI 1.0.100. The startup notice says an administrator-managed setting replaced the selected model with auto, while a subsequent manual model selection succeeds. I have not separately verified whether the existing --model flag is affected in this scenario; please clarify existing support if it already handles this workflow.