custom providers do not inherit provider loaders #18486

Description

Providers configured in opencode.json inherit config values, but they do not inherit the referenced provider's auth/model loader behavior. That breaks aliased providers that expect built-in provider SDK options or dynamic model loading.

Plugins

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OpenCode version

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Steps to reproduce

1. Configure a custom provider:

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "my-github-copilot": {
      "name": "my-github-copilot",
      "npm": "@ai-sdk/github-copilot"
    }
  }
}

The auth will default to api-key even though @ai-sdk/github-copilot support oAuth natively. The expectation is that "my-github-copilot" should allow me to chose between oAuth and api-key.

Opening this issue for my PR that adds auth-provider that would solve the issue.

Screenshot and/or share link

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Operating System

macOS

Terminal

Ghostty