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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pi-hermes-memory/src/subagent-mode.ts

@@ -0,0 +1,58 @@
+/**
+ * subagent-mode.ts — subagent sessions are read-only for memory and skill
+ * writes. All automatic write-related work (background review, correction
+ * detection, session flushes, session indexing) is skipped as well.
+ *
+ * A child session is identified by the `<active_agent name="..."/>` identity
+ * tag that pi-subagents stamps into every child prompt
+ * (permission-system-bridge.ts): the tag arrives via
+ * `systemPromptOptions.customPrompt`. The tag is only trusted on that field —
+ * never in the rendered system prompt, which a main session can legitimately
+ * contain through project docs.
+ *
+ * Membership is tracked per session by its sessionManager identity: the same
+ * process hosts the parent and its children concurrently, so a global boolean
+ * would let a child's turn flip the parent's session into read-only mid-turn.
+ */
+import type { BeforeAgentStartEvent, ExtensionContext } from "@earendil-works/pi-coding-agent";
+
+/** pi-subagents `<active_agent name="X"/>` identity tag contract. */
+const ACTIVE_AGENT_TAG = /<active_agent\s+name=["'][^"']+["'][^>]*>/i;
+const LEADING_ACTIVE_AGENT_TAG = /^\s*<active_agent\s+name=["'][^"']+["'][^>]*>/i;
+
+export const SUBAGENT_WRITE_DISABLED_MESSAGE =
+  "Memory and skill writes are disabled in subagent sessions; include this in your final report so the delegating agent can decide.";
+
+const subagentSessionManagers = new WeakSet<object>();
+
+/** Same regex as the pi-subagents `<active_agent>` contract, applied only to customPrompt. */
+export function isSubagentCustomPrompt(customPrompt: string | undefined | null): boolean {
+  return typeof customPrompt === "string" && ACTIVE_AGENT_TAG.test(customPrompt);
+}
+
+/**
+ * before_agent_start hook: authoritative detection via the event's
+ * systemPromptOptions.customPrompt.
+ */
+export function markSubagentSession(event: BeforeAgentStartEvent, ctx: ExtensionContext): void {
+  if (isSubagentCustomPrompt(event.systemPromptOptions?.customPrompt)) {
+    subagentSessionManagers.add(ctx.sessionManager);
+  }
+}
+
+/**
+ * session_start runs before before_agent_start; detect tag-leading prompts
+ * early so session_start writes (skill migration, root setup) can be skipped.
+ * A main prompt cannot lead with the tag — without customPrompt the rendered
+ * prompt always opens with the base preamble.
+ */
+export function markSubagentSessionFromPrompt(systemPrompt: string, ctx: ExtensionContext): void {
+  if (LEADING_ACTIVE_AGENT_TAG.test(systemPrompt)) {
+    subagentSessionManagers.add(ctx.sessionManager);
+  }
+}
+
+export function isSubagentSession(ctx: unknown): boolean {
+  const sm = (ctx as { sessionManager?: unknown } | undefined | null)?.sessionManager;
+  return typeof sm === "object" && sm !== null && subagentSessionManagers.has(sm);
+}