feat(agents): add agent override configuration support

- 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

src/agents/index.ts

@@ -12,3 +12,6 @@ export const builtinAgents: Record<string, AgentConfig> = {
   "frontend-ui-ux-engineer": frontendUiUxEngineerAgent,
   "document-writer": documentWriterAgent,
 }
+
+export * from "./types"
+export { createBuiltinAgents } from "./utils"