feat(agent): agent.await suspends the session instead of holding a worker slot

When children are still running, agent.await records the wait, closes
the session (the tool call stays pending in the JSONL) and drive returns
suspended. runAgent waits with waitForFact for the children to settle,
the deadline, or cancellation, then reopens the session; agent.await is
replay-safe, so resume replays it and the model gets the real result.

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

apps/server/test/e2e/e11-subagents.test.ts

@@ -183,6 +183,12 @@ describe.skipIf(!hasPg)("E11 Pi 子代理(agent.spawn / agent.await)", () => {
     )!;
     const lastCompleted = events.filter((x) => x.type === "subagent_completed").at(-1)!;
     expect(Number(lastCompleted.seq)).toBeLessThan(Number(awaitEnd.seq));
+    // agent.await 挂起(不占 worker):会话关闭 → waitForFact 等子会话 → 重开会话重放 agent.await
+    expect((awaitEnd.payload as Any).recovery).toBe(true);
+    expect(events.filter((x) => x.type === "run_started").map((x) => (x.payload as Any).resumed)).toEqual([
+      false,
+      true,
+    ]);
     // SSE:父会话 runId 上能看到子会话提示
     expect(seen.some((x) => x.payload.runId === parentId && x.payload.capabilityId === "agent.spawn")).toBe(true);