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]>

apps/server/test/e2e/e2-agent-delegate.test.ts

@@ -42,8 +42,8 @@ describe.skipIf(!hasPg)("E2 Agent 委托闭环(fake 模型 + 真实能力/授权
     return r.body.data.id as string;
   };
 
-  it("启动断言:15 个能力全部注册,并在能力目录里可见", async () => {
-    expect(e.booted.ctx.capabilities.list()).toHaveLength(15);
+  it("启动断言:17 个能力全部注册,并在能力目录里可见", async () => {
+    expect(e.booted.ctx.capabilities.list()).toHaveLength(17);
     const r = await e.api("GET", "/capabilities");
     expect(r.body.data.items.map((c: { id: string }) => c.id).sort()).toEqual(
       e.booted.ctx.capabilities