feat(agents): add build agent prompt extension and configuration override support

- Add BUILD_AGENT_PROMPT_EXTENSION for orchestrator-focused main agent behavior
- Introduce OverridableAgentName type to allow build agent customization
- Update config schema to support build agent override in oh-my-opencode.json
- Inject orchestration prompt into build agent's system prompt

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/agents/index.ts

@@ -17,3 +17,4 @@ export const builtinAgents: Record<string, AgentConfig> = {
 
 export * from "./types"
 export { createBuiltinAgents } from "./utils"
+export { BUILD_AGENT_PROMPT_EXTENSION } from "./build"