Support same model naming convention as github copilot in vscode

### Describe the bug

Currently, cli naming convention for models is different than vscode github copilot or claude code itself

### Affected version

1.0.22

### Steps to reproduce the behavior

1. in any agent, for model use name: opus
2. in cli, use /agent
3. you will see a warning message model "opus" is not available; will use current model instead

### Expected behavior

Support naming model same as github copilot in vscode

### Additional context

_No response_

Start by tracing the /agent flow that validates model names and produces the unavailable-model warning. Compare the accepted naming convention with the names used by GitHub Copilot in VS Code and Claude Code; done means the reported model alias is recognized without falling back to the current model.

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