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/db/migrations/0029_agent_run_parent.ts

@@ -0,0 +1,50 @@
+import type { Kysely } from "kysely";
+import { execDdl, oneOf } from "../src/migration-helpers.ts";
+
+// W7-B Pi 子代理(agent.spawn):agent_run 已有 parent_run_id(0010),这里补派生深度与派生理由;
+// agent_run_event 增加 subagent_spawned / subagent_event / subagent_completed(子会话事件镜像进父会话事件流)。
+
+const OLD_TYPES = [
+  "run_started",
+  "text_delta",
+  "thinking_delta",
+  "tool_call_start",
+  "tool_call_end",
+  "tool_call_rejected",
+  "compaction",
+  "run_completed",
+  "run_failed",
+];
+const NEW_TYPES = [...OLD_TYPES, "subagent_spawned", "subagent_event", "subagent_completed"];
+
+const swap = (values: readonly string[]) => `
+alter table agent_run_event drop constraint agent_run_event_type_check;
+alter table agent_run_event add constraint agent_run_event_type_check ${oneOf("type", values)};
+`;
+
+// biome-ignore lint/suspicious/noExplicitAny: 迁移不依赖应用表类型
+export async function up(db: Kysely<any>): Promise<void> {
+  await execDdl(
+    db,
+    `
+alter table agent_run add column depth integer not null default 0 check (depth >= 0);
+alter table agent_run add column spawn_reason text;
+create index agent_run_parent_idx on agent_run (parent_run_id, created_at) where parent_run_id is not null;
+${swap(NEW_TYPES)}
+`,
+  );
+}
+
+// biome-ignore lint/suspicious/noExplicitAny: 同上
+export async function down(db: Kysely<any>): Promise<void> {
+  await execDdl(
+    db,
+    `
+delete from agent_run_event where type in ('subagent_spawned', 'subagent_event', 'subagent_completed');
+${swap(OLD_TYPES)}
+drop index if exists agent_run_parent_idx;
+alter table agent_run drop column spawn_reason;
+alter table agent_run drop column depth;
+`,
+  );
+}