Include previous model in /model change confirmation

[Ghost93]

Describe the feature or problem you'd like to solve

Show both the previous and new model in the model switch confirmation

Proposed solution

Today, when I switch models, Copilot CLI shows a message like:

Model changed to: gpt-5.4 (xhigh)

That tells me the new state, but not what it changed from. When switching models frequently, or when multiple sessions are open, that makes it harder to confirm whether the right change actually happened.

Please include both the source and destination model in the confirmation message. For example:

Model changed from claude-sonnet-4.6 to gpt-5.4 (xhigh)

This would make the change immediately clear without adding much noise, and would be especially helpful when:

• switching models often during a session
• changing reasoning effort on the same base model
• multiple sessions are open and model changes become harder to track

If useful, a more specific message for same-model effort-only changes could also help, for example:

Reasoning effort changed for gpt-5.4 from high to xhigh

Example prompts or workflows

• I am in the middle of a session and switch from a fast model to a heavier reasoning model for one difficult prompt. Seeing both models confirms I switched exactly as intended.
• I switch models a few times during a long session. A from X to Y message makes the scrollback much easier to understand later.
• I have two Copilot CLI sessions open. If the active model changes unexpectedly, showing both the old and new values makes the change much easier to notice and diagnose.
• I change only reasoning effort on the same model. The confirmation should make that precise change obvious

Additional context

Current output I see:

Model changed to: gpt-5.4 (xhigh)

I checked existing issues and found related items, but not one for this exact request - #1355, #1531

This feels like a small UX change, but it would improve clarity and confidence for people who switch models often.