feat(pi-subagents): 重新启用子 Agent 并使用独立的子 Agent 提示词

- 根包恢复 file:./pi-subagents 依赖与 extensions/subagents.ts 加载
  （排在 permission-system 之后、Tool Search 之前），并刷新根锁文件；
  Agent / get_subagent_result / steer_subagent 权限设为 allow，
  子 Agent 内的实际操作仍逐个经权限系统并转发主会话。
- 新增 config/prompt/subagent/：执行者角色 preamble 与组合包自带的
  general-purpose / Explore / Plan 定义（替换上游内置，去掉 bash find
  冲突规则与写死的 Haiku 模型）。
- pi-subagents 新增 basePrompt / bundledAgentsDir / excludedExtensionPaths
  选项：子 Agent 不再嵌入父会话提示词，身份标记置于开头；子会话排除
  pi-ask-user。未传选项时保持上游行为。
- system-prompt.ts：customPrompt 自带开场白时不再覆盖（修复子 Agent 角色
  与 <active_agent> 身份被抹掉）；仅凭 customPrompt 上的 <active_agent>
  识别子会话，routing 规则支持 scope，sections 支持 subagentExclude
  （子会话去掉 memory_policy 与 chrome_profile_bridge）。
- pi-hermes-memory：子会话只读，记忆/skill 写入工具返回错误，后台整理、
  纠错检测、会话 flush 与索引写入全部跳过。

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

pi-subagents/src/nested-tools.ts

@@ -15,7 +15,7 @@ import {
   resolveEnabledTypeIn,
   resolveTypeIn,
 } from "./agent-types.js";
-import { loadCustomAgents } from "./custom-agents.js";
+import { loadBundledAgents, loadCustomAgents } from "./custom-agents.js";
 import { isolationParam, resolveAgentInvocationConfig } from "./invocation-config.js";
 import { resolveModel } from "./model-resolver.js";
 import { checkModelScope } from "./model-scope.js";
@@ -100,6 +100,8 @@ export interface NestedToolContext {
   allowedSubagents: "all" | string[];
   /** Root used for agent/config discovery; may differ from the agent's working directory. */
   configCwd: string;
+  /** Bundle-shipped agent .md directory — registered as embedded defaults, like the top-level registry. */
+  bundledAgentsDir?: string;
 }
 
 function textResult(text: string, isError = false) {
@@ -145,7 +147,10 @@ export function createNestedSubagentTools(context: NestedToolContext): ToolDefin
   // Agents resolve from a registry built for THIS branch's config root (under
   // worktree isolation, the copy). Never via registerAgents — that is
   // process-global state shared with the main session and every other agent.
-  const loadRegistry = () => buildAgentRegistry(loadCustomAgents(context.configCwd));
+  const loadRegistry = () => buildAgentRegistry(
+    loadCustomAgents(context.configCwd),
+    context.bundledAgentsDir === undefined ? undefined : loadBundledAgents(context.bundledAgentsDir),
+  );
   const allowedTypesIn = (registry: Map<string, AgentConfig>): Set<string> | undefined =>
     context.allowedSubagents === "all"
       ? undefined