Mismatched model names between cli and vs code

### Describe the bug

There is an inconsistency in how custom agent model names are parsed/validated between the Copilot CLI and the VS Code Copilot Chat extension.

When configuring a custom agent with the model name set to "GPT-5.3-Codex (copilot)", VS Code Copilot Chat accepts it and functions perfectly. However, running the Copilot CLI throws a warning stating the model is not available.

It appears the CLI has a stricter or outdated list of acceptable model aliases like "gpt-5.3-codex" instead of the capitalized display names VS Code accepts.

```sh
The following agents have warnings:
• /Users/my-user/.config/.copilot/agents/coder.agent.md: model "GPT-5.3-Codex (copilot)" is not available; will use current model instead
```

### Affected version

GitHub Copilot CLI 0.0.420

### Steps to reproduce the behavior

1. Create a custom agent configuration file at ~/.config/.copilot/agents/coder.agent.md.
2. Add the following to the file:
```markdown
---
name: coder
model: "GPT-5.3-Codex (copilot)"
---
You are a coding assistant.
````
3. Open VS Code Copilot Chat and verify the agent works without errors.
4. Open a terminal and run github-copilot-cli
5. Trigger the custom agent (type /agent).

### Expected behavior

Both the Copilot CLI and VS Code Copilot Chat should share the same validation logic and allowed aliases for model names. If a model string is valid in VS Code, the CLI should accept it without throwing a fallback warning.

### Additional context

_No response_