OAuth providers or pre-configured providers in opencode

### Problem Statement

Hello,
The supported, preconfigured providers are primarily APIs. Therefore, it is not possible to use OpenAI, OpenCode Go/Zen, or other subscriptions.

### Proposed Solution

It would be great to be able to use providers like OpenCode Go/Zen or OpenAI Pro via OAuth (similar to OpenCode).

### Alternatives Considered

Alternatively, it would be helpful to use the providers configured in OpenCode directly, so we don't have two sources of LLMs.

### Affected Area

Configuration

### Additional Context

_No response_

No files, tests, or entry points are identified. Start by tracing configuration and supported-provider handling, then determine whether OAuth for OpenAI and OpenCode Go/Zen or reuse of OpenCode's provider configuration is feasible; done means one consistent way to use those subscriptions.

Written by the indexing model from the issue text.