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-hermes-memory/src/index.ts

@@ -61,6 +61,7 @@ import { AGENT_ROOT } from "./paths.js";
 import { isDatabaseMigrationPending } from "./extension-root-migration.js";
 import { measureLifecycle, measureLifecycleSync } from "./lifecycle-timing.js";
 import { createMemoryInitializer, withMemoryInitialization, type EnsureMemoryReady } from "./memory-initialization.js";
+import { isSubagentSession, markSubagentSession, markSubagentSessionFromPrompt } from "./subagent-mode.js";
 
 export function resolveProjectSkillDiscovery(
   skillStore: SkillStore,
@@ -195,8 +196,12 @@ export default function (pi: ExtensionAPI) {
 
   const initialization = createMemoryInitializer(async (ctx) => {
     const timingPrefix = lazy ? "memory-init" : "session-start";
-    if (lazy) migrateLegacyProjectMemoryDirs(agentRoot, config.projectsMemoryDir);
-    if (!persistenceInitialized) {
+    // Subagent sessions are read-only: skip the write side of initialization
+    // (legacy-dir migration, markdown↔SQLite sync, pruning, backfill) while
+    // still loading the store so memory_search/session_search keep working.
+    const subagent = isSubagentSession(ctx);
+    if (lazy && !subagent) migrateLegacyProjectMemoryDirs(agentRoot, config.projectsMemoryDir);
+    if (!persistenceInitialized && !subagent) {
       try {
         await measureLifecycle(`${timingPrefix}.persistence-sync`, async () => {
           // Startup reconciles the global files plus the current project only:
@@ -290,18 +295,25 @@ export default function (pi: ExtensionAPI) {
   // Skills and pinned instructions must be available even without a lookup.
   pi.on("session_start", async (_event, ctx) => {
     sessionContext = ctx;
+    // session_start runs before before_agent_start; a child's rendered prompt
+    // leads with the <active_agent> tag, so detect it here early enough to
+    // skip the session_start writes below.
+    markSubagentSessionFromPrompt(ctx.getSystemPrompt?.() ?? "", ctx);
     // Pinned directives must not depend on migration/SQLite being healthy.
     if (standingStore) await standingStore.load();
     if (!lazy) await ensureMemoryReady(ctx);
     refreshSkillProjectContext(ctx.cwd);
-    await skillStore.migrateLegacySkills();
-    await skillStore.ensureDiscoveredRoots();
+    if (!isSubagentSession(ctx)) {
+      await skillStore.migrateLegacySkills();
+      await skillStore.ensureDiscoveredRoots();
+    }
   });
 
   registerProjectSkillDiscoveryHandler(pi, skillStore, config.projectsMemoryDir);
 
   // ── 2. Inject memory policy by default; legacy mode keeps full frozen memory blocks ──
-  pi.on("before_agent_start", async (event, _ctx) => {
+  pi.on("before_agent_start", async (event, ctx) => {
+    markSubagentSession(event, ctx);
     const promptContext = await buildPromptContext(config, store, projectStoreRef(), projectNameRef(), standingStore);
     if (!promptContext) return;
 
@@ -400,6 +412,7 @@ export default function (pi: ExtensionAPI) {
 
   // ── 10. Live session indexing ──
   pi.on("message_end", async (_event, ctx) => {
+    if (isSubagentSession(ctx)) return;
     // Cold lazy sessions remain in Pi's JSONL files and are backfilled on use.
     if (lazy && !initialization.isReady()) return;
     scheduleLiveSessionIndex(dbManager, ctx.sessionManager, {
@@ -432,6 +445,7 @@ export default function (pi: ExtensionAPI) {
       return;
     }
     try {
+      if (isSubagentSession(ctx)) return;
       measureLifecycleSync("shutdown.active-index", () => {
         const sessionFile = ctx.sessionManager.getSessionFile();
         if (sessionFile && fs.existsSync(sessionFile)) {