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/web/app/components/agent/agent-run-panel.tsx

@@ -34,6 +34,7 @@ import {
   RunStatusBadge,
   runPlanIds,
 } from "./agent-conversation.tsx";
+import { SubagentTree } from "./agent-subagent-tree.tsx";
 import { useAgentStream } from "./use-agent-stream.ts";
 
 const TRIGGER_LABEL = { user: "用户发起", event: "事件触发", schedule: "定时", subagent: "子代理" } as const;
@@ -181,11 +182,22 @@ export function AgentRunPanel({
   const audit = useRunInvocationAudit(runId, toolCount > 0, toolCount);
   const approvals = useRunApprovals(runPlanIds(live.items, run.data?.report ?? null));
   // run 状态变化(queued → running → 终态)经 SSE 推送:到达即刷新快照,不等 3 秒轮询
+  // 子会话:本会话收到子会话提示(agent.tool_call / agent.spawn)或任一子会话 / 父会话状态变化时刷新会话树
+  const related = new Set([
+    runId,
+    ...(run.data?.children ?? []).map((c) => c.id),
+    ...(run.data?.parent ? [run.data.parent.id] : []),
+  ]);
   useSse(
-    [EventType.agentRunStatus],
+    [EventType.agentRunStatus, EventType.agentToolCall],
     (e) => {
-      if ((e.payload as { runId?: string } | null)?.runId === runId)
-        void qc.invalidateQueries({ queryKey: agentKeys.run(runId) });
+      const p = e.payload as { runId?: string; capabilityId?: string } | null;
+      if (!p?.runId) return;
+      const hit =
+        e.type === EventType.agentRunStatus
+          ? related.has(p.runId)
+          : p.runId === runId && p.capabilityId === "agent.spawn";
+      if (hit) void qc.invalidateQueries({ queryKey: agentKeys.run(runId) });
     },
     !agentMockEnabled(),
   );
@@ -245,6 +257,8 @@ export function AgentRunPanel({
         <BudgetBar run={r} live={live} />
       </div>
 
+      <SubagentTree snapshot={run.data} />
+
       {mode === "plan_only" ? <PlanSteps live={live} active={streaming} /> : null}
 
       {streaming && sseStatus === "reconnecting" ? (