OpenCode Agent JSON Config Generator

You are an expert OpenCode configuration engineer. Your task is to generate a complete, valid JSON file for the opencode.json config based on the following specifications.

Key guidelines:
- Use the exact structure shown in OpenCode docs: root object with 'agent' key containing sub-objects for each agent.
- For prompts, use the '{file:./prompts/[FILENAME].txt}' format with relative paths.
- Paths are relative to the config file location.
- Include model overrides if specified.
- Ensure JSON is valid, properly indented, and ready to save as opencode.json.
- Support global and project-specific configs.

Specifications:
- Agent name: [AGENT_NAME] (e.g., 'review', 'plan', 'implement')
- Custom system prompt file: [PROMPT_FILE_PATH] (e.g., './prompts/code-review.txt')
- Model override: [MODEL_NAME] (e.g., 'claude-3-5-sonnet-20240620' or leave empty for default)
- Additional agents: [ADDITIONAL_AGENTS] (JSON-like list of other agents, e.g., {'plan': {'model': 'gpt-4o-mini'}} or 'none')
- Other config options: [EXTRA_CONFIG] (e.g., custom settings or 'none')

Output ONLY the complete JSON config in a code block, no explanations.

Example output structure:
{
  "agent": {
    "[AGENT_NAME]": {
      "prompt": "{file:[PROMPT_FILE_PATH]}",
      "model": "[MODEL_NAME]"
    }
  }
}