Add config/flag/env option to disable claude code compatibility loading

### Describe the feature or problem you'd like to solve

I use both Copilot CLI and Claude Code simultaneously, and Copilot CLI's automatic loading from .claude/ directories (agents, skills) interferes with my intentionally separate setups for each tool.

### Proposed solution

Reference to [loading order of Copilot CLI](https://docs.github.com/en/copilot/reference/cli-plugin-reference#loading-order-and-precedence), the CLI loading agents and skills from `.claude` directory. Add a opt-out config to `config.json` :
```json
{
"claude": {
"agents": false,
"skills": false,
"commands": false
}
}
```

### Example prompts or workflows

_No response_

### Additional context

- [Copilot CLI plugin loading order](https://docs.github.com/en/copilot/reference/cli-plugin-reference#loading-order-and-precedence)
- [Opencode env var to disable Claude Code skills/commands](https://github.com/anomalyco/opencode/issues/12604)