chore: Update model version in CLI usage section

[Cadienvan]

I literally just installed the Copilot CLI to test Sonnet 4.5 but 4 was set by default.

[Copilot]

Pull Request Overview

This PR corrects outdated documentation in the README to reflect the actual default model used by the Copilot CLI, changing from Claude Sonnet 4.5 to Claude Sonnet 4.

• Updates the default model reference from Claude Sonnet 4.5 to Claude Sonnet 4
• Adjusts the list of alternative models to include Claude Sonnet 4.5 instead of Claude Sonnet 4

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[williammartin]

Well, 4.5 is the default but sometimes it may not be accessible to the user. We could probably be clearer about this, but the PR as-is is not correct, so I'm going to close it because I suspect the nuances of when it is or isn't accessible might require a bit more digging. Thanks for trying to improve things anyway!