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/types.ts

@@ -1,6 +1,6 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
 
-export type AgentName =
+export type BuiltinAgentName =
   | "oracle"
   | "librarian"
   | "explore"
@@ -8,6 +8,12 @@ export type AgentName =
   | "document-writer"
   | "multimodal-looker"
 
+export type OverridableAgentName =
+  | "build"
+  | BuiltinAgentName
+
+export type AgentName = BuiltinAgentName
+
 export type AgentOverrideConfig = Partial<AgentConfig>
 
-export type AgentOverrides = Partial<Record<AgentName, AgentOverrideConfig>>
+export type AgentOverrides = Partial<Record<OverridableAgentName, AgentOverrideConfig>>