feat(agent): 实现委派工具重构，支持子代理和代理团队模式

- 新增 spawn_subagent 和 spawn_agent_team 工具，替代原有的 spawn 工具
- 重构 DelegationManager 以支持单个子代理和代理团队两种委派模式
- 更新系统提示词中的委派策略说明，明确使用场景和区别
- 添加技能上下文传递功能，确保委派任务遵循指定技能
- 实现代理内部的受控下游委派机制，防止无限嵌套
- 更新工具注册和上下文设置逻辑以适配新架构

app-instance/backend/nanobot/agent/context.py

@@ -86,15 +86,12 @@ Skills with available="false" need dependencies installed first - you can try in
 {skills_summary}""")
 
         if self.agent_registry:
-            # 把可委派 agent 目录加入 system prompt，模型才知道 `spawn` 能调用谁。
-            agents_summary = self.agent_registry.build_agents_summary()
-            if agents_summary:
-                parts.append(f"""# Available Agents
+            parts.append("""# Delegation Tools
 
-The following agents can be delegated to via the `spawn` tool.
-Use `target` for a single agent and `targets` for a group.
-
-{agents_summary}""")
+Use `spawn_subagent` when the task should go to one delegated worker.
+Use `spawn_agent_team` when the task should be explored in parallel by multiple workers.
+At the top level, you do not need to choose concrete downstream agents.
+Use the `skills` argument when the delegated worker or team must follow specific skills.""")
 
         if execution_context:
             # `execution_context` 用于 cron / system task 这类“不是普通用户消息”的额外运行说明。
@@ -142,6 +139,14 @@ Reply directly with text for conversations. Only use the 'message' tool to send
 - If a tool call fails, analyze the error before retrying with a different approach.
 - Do not write directly into `{workspace_path}/skills`; new or updated skills must go through the review flow before activation.
 
+## Delegation Policy
+- Solve simple tasks yourself when the work is short, direct, and does not benefit from delegation.
+- Delegate only when the task is complex, multi-step, time-consuming, or benefits from specialized/parallel work.
+- Use `spawn_subagent` for one focused delegated worker when only the final result matters.
+- Use `spawn_agent_team` when multiple agents should explore the task in parallel, compare findings, or work across separate areas.
+- Do not delegate by default if you can complete the task reliably in the current turn.
+- Do not create or modify persistent local sub-agents unless the user explicitly asks for a reusable long-lived worker.
+
 ## Memory
 - Remember important facts: write to {workspace_path}/memory/MEMORY.md
 - Recall past events: grep {workspace_path}/memory/HISTORY.md"""