- Add AgentName, AgentOverrideConfig, AgentOverrides types
- Implement createBuiltinAgents with disabled_agents and overrides support
- Support oh-my-opencode.json config for:
- disabled_agents: disable specific built-in agents
- agents: override model, temperature, tools, permission per agent
- Tools and permission objects are shallow-merged with base config
- Export types for external consumers
- Update README with agent override documentation
