When configuring custom headers (including User-Agent) for a provider or model
in opencode.jsonc, the headers are not actually sent with the API requests.

For example, this configuration does not work:

```json
    {
      "provider": {
        "myprovider": {
          "models": {
            "my-model": {
              "headers": {
                "User-Agent": "MyCustomAgent/1.0"
              }
            }
          }
        }
      }
    }
```

In packages/opencode/src/provider/provider.ts, the getSDK function correctly
merges model.headers into options["headers"]:

```ts
if (model.headers)
      options["headers"] = {
        ...options["headers"],
        ...model.headers,
      }
```

However, in the custom fetch function, opts.headers comes from the init
parameter passed by the AI SDK, not from options["headers"]. This means the
configured headers are never actually included in the request.


Solution

Update the custom fetch function to merge options["headers"] into opts.headers:

```ts
        opts.headers = {
          ...(typeof opts.headers === 'object' ? opts.headers : {}),
          ...options["headers"],
        };
```