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/agent/src/settings.ts

@@ -30,7 +30,7 @@ export const AgentSettingsSchema = z.object({
       threshold: z.number().min(0.5).max(0.95).default(0.8),
     })
     .default({ mode: "threshold", threshold: 0.8 }),
-  /** v0.1 未实现子代理;false 时忽略,true 也只记录(由 agent.delegate 能力后续实现) */
+  /** 子代理开关(仅记录);派生由服务端 agent.spawn 能力实现,开关与并发/深度上限在服务端校验 */
   subagentsEnabled: z.boolean().default(false),
   maxToolCalls: z.number().int().min(1).default(40),
   maxDurationMs: z