openrouter integration

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

setup openrouter api & choose their models

### Proposed solution

tried claude code with openrouter, got stuck while keep requestin/consuming the token.
i love to work with copilot-cli, openrouter can be used in copilot chat, i think it could be done in copilot-cli

or follow this https://openrouter.ai/docs/guides/coding-agents/claude-code-integration
thank you.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start with the linked OpenRouter coding-agents documentation, then inspect copilot-cli's existing model configuration and request or token-consumption flow. The issue provides no file or test entry point; done would mean a supported OpenRouter setup, selectable OpenRouter models, and verified request consumption without the reported loop.

索引モデルが issue の本文から書いたものです。