add opencode-go provider

[HanSoBored]

I have an opencode-go subscription, but the opencode-go provider has not been added. If possible, add opencode-go to the provider.

[vernonstinebaker]

This may already be covered by the custom OpenAI-compatible provider work, depending on how opencode-go exposes its API.

NullHub now has a generic OpenAI-compatible path that accepts a custom base URL instead of requiring a first-class built-in provider: #33 adds OpenAI-compatible provider support with a base_url in the UI/API #34 adds the probe-models flow so a custom endpoint can be validated and its model list fetched before saving. I've confirmed locally: opencode-go works through the generic OpenAI-compatible provider path.

I was able to add it as a provider and spin up an instance successfully using the custom base_url flow.

That means this issue is addressed by the OpenAI-compatible provider and doesn't require a named provider entry. Once #33 and #34 are merged you should be good to go.