Implement automatic retry with exponential backoff when Anthropic returns {"type": "overloaded_error"} responses, by checking the error message on overloaded_error inclusion.

Changes

• Add ServiceOverloadedError type to properly categorize these errors
• Implement configurable retry logic with exponential backoff and jitter
• Add retry configuration schema with sensible defaults (20 retries, 30s max delay)

Configuration

We can now configure retry behavior in opencode.json:

{
  "retry": {
    "maxRetries": 20,
    "initialDelay": 1000,
    "maxDelay": 30000
  }
}

Closes #833