fix(opencode): default header timeout to five minutes

[rekram1-node]

Sets the default timeout for waiting for provider response headers to five minutes (300,000 ms), so requests don't wait indefinitely before a response starts. The timer stops once headers arrive and does not limit the streamed response body. OpenAI's existing five-minute header timeout is unchanged.

To disable it for a provider, set headerTimeout to false in your project's opencode.json or global ~/.config/opencode/opencode.json:

{
  "provider": {
    "anthropic": {
      "options": {
        "headerTimeout": false
      }
    }
  }
}

Replace anthropic with your provider. To change the timeout instead, use a positive number of milliseconds, such as "headerTimeout": 600000 for ten minutes. Omit the setting to use the five-minute default.