feat(agent): Pi subagents via agent.spawn / agent.await

- agent_run gains depth / spawn_reason (0029); agent_run_event types
  subagent_spawned / subagent_event / subagent_completed
- agent.spawn (propose, medium, replay never): allowlist ⊆ parent, mode not
  above parent, budget carved from parent's remainder, idempotent per run+input;
  settings.pi.subagents {enabled, maxConcurrent, maxDepth} (subagentsEnabled kept as alias)
- child runs claim a concurrency slot under an advisory lock (waitForFact while
  queued); agent.await polls business tables without calling the model
- parent abort cascades to all descendants; parent runtime budget counts
  descendant usage; child events mirrored into the parent stream by parent seq
- authorize caps every child call by all ancestors' allowlists/kinds
- session page shows parent link, inherited info and the subagent tree
- e2e E11; runbook section on subagents

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/capability.ts

@@ -66,6 +66,8 @@ export const CAPABILITY_IDS = [
   "policy.preview",
   "policy.apply",
   "inventory.query",
+  "agent.spawn",
+  "agent.await",
 ] as const;
 export const KnownCapabilityId = z.enum(CAPABILITY_IDS);
 export type KnownCapabilityId = z.infer<typeof KnownCapabilityId>;
@@ -271,6 +273,30 @@ export const CAPABILITY_CATALOG: Record<KnownCapabilityId, CapabilityDescriptor>
     true,
     false,
   ),
+  // 子代理(设置 pi.subagents 开启后才提供给模型):类别 propose——只建有界子会话,不产生外部副作用,
+  // "先给方案"的会话也能派生(子会话同样只给方案);授权不高于父会话,由服务端校验
+  "agent.spawn": d(
+    "agent.spawn",
+    "派生子会话",
+    "把一个有界子目标交给子会话处理:能力白名单须是本会话白名单的子集,预算从本会话剩余预算里划拨,模式不高于本会话;返回子会话 runId。await=true 时等它结束并返回汇报",
+    "propose",
+    "AgentSpawn",
+    "organize.execute",
+    "medium",
+    true,
+    false,
+  ),
+  "agent.await": d(
+    "agent.await",
+    "等待子会话",
+    "等待本会话派生的子会话结束(等待期间不调用模型),返回各自状态、用量与汇报摘要;超时仍未结束的标为 pending",
+    "read",
+    "AgentAwait",
+    "read",
+    "low",
+    true,
+    false,
+  ),
 };
 
 function d(