Feature Request: Add OpenCode as a Provider in GitHub Copilot

### Feature summary

Please add support for OpenCode as a model provider within the GitHub Copilot application, similar to the existing support for other third-party AI providers and local/custom model endpoints.

### What problem are you trying to solve?

Many developers prefer using OpenCode because it offers:

* Greater flexibility in model selection and deployment.
* The ability to use self-hosted or custom AI models.
* More control over privacy, costs, and data handling.
* An open ecosystem that can be tailored to individual workflows.

Currently, there is no straightforward way to use OpenCode directly within GitHub Copilot, which limits developers who have standardized on OpenCode for their AI infrastructure.

### Proposed solution

Add **OpenCode** as a configurable provider in GitHub Copilot, allowing users to:

1. Select OpenCode from the list of available providers.
2. Configure:

* Server URL / endpoint
* API key or authentication method
* Model selection
* Optional provider-specific settings
3. Use OpenCode-powered completions, chat, and agent capabilities through the existing GitHub Copilot interface.

## Example Configuration

```json
{
"provider": "opencode",
"endpoint": "https://your-opencode-server.com",
"apiKey": "your-api-key",
"model": "your-model"
}
```

## Benefits

* Increases flexibility and developer choice.
* Enables self-hosted and privacy-focused AI workflows.
* Expands GitHub Copilot's compatibility with the growing open-source AI ecosystem.
* Allows teams already using OpenCode to adopt GitHub Copilot without changing their existing infrastructure.

## Additional Context

OpenCode is gaining adoption among developers who want an open and customizable AI backend. Supporting it as a provider would make GitHub Copilot more extensible and appealing to teams with diverse AI deployment requirements.

Thank you for considering this enhancement.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

There is already a project on GitHub which is [VS Code extension which integrates OpenCode as provider](https://github.com/ltmoerdani/opencode-copilot-chat) in the Copliot Chat. It might act as a referece on how the implementation may work.

I see there is a [bug report about a similar implementation](https://github.com/github/app/issues/1409) but it focuses on the OpenAI Compatible endpoint, while I suggest to implement OpenCode as built-in provider, like the VS Code extension is doing it.

On a side note, worth mentioning that the extension registeres OpenCode for the Local sessions in the Agent Window in VS Code but not yet in the Remote.

まず、リンクされている VS Code 拡張機能 opencode-copilot-chat と GitHub issue #1409 を確認し、既存の provider と OpenAI-compatible endpoint に関するコンテキストを把握します。次に、Copilot アプリの provider 統合ポイントを特定し、configuration、completions、chat、agent capabilities がどのように表現されているかを確認します。既存のアプリケーション workflow を通じて、OpenCode を設定可能な provider として利用できるようになれば完了です。

索引モデルが issue の本文から書いたものです。