feat(background-agent): add wait-for-task-session helper

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/background-agent/wait-for-task-session.ts

@@ -0,0 +1,68 @@
+import { getTimingConfig } from "../../tools/delegate-task/timing"
+import type { BackgroundTaskStatus } from "./types"
+
+type SessionWaitTerminalStatus = Extract<BackgroundTaskStatus, "error" | "cancelled" | "interrupt">
+type AbortSignalLike = { aborted: boolean }
+
+interface TaskReader {
+  getTask(taskID: string): { sessionID?: string; status?: BackgroundTaskStatus } | undefined
+}
+
+export interface WaitForTaskSessionIDOptions {
+  timeoutMs?: number
+  intervalMs?: number
+  signal?: AbortSignalLike
+}
+
+function isTerminalStatus(status: BackgroundTaskStatus | undefined): status is SessionWaitTerminalStatus {
+  return status === "error" || status === "cancelled" || status === "interrupt"
+}
+
+function waitForInterval(intervalMs: number): Promise<void> {
+  return new Promise(resolve => {
+    const scheduler = globalThis as { setTimeout: (handler: () => void, timeout?: number) => unknown }
+    scheduler.setTimeout(resolve, intervalMs)
+  })
+}
+
+export async function waitForTaskSessionID(
+  manager: TaskReader,
+  taskID: string,
+  options: WaitForTaskSessionIDOptions = {}
+): Promise<string | undefined> {
+  const timing = getTimingConfig()
+  const timeoutMs = options.timeoutMs ?? timing.WAIT_FOR_SESSION_TIMEOUT_MS
+  const intervalMs = options.intervalMs ?? timing.WAIT_FOR_SESSION_INTERVAL_MS
+
+  if (options.signal?.aborted) {
+    return undefined
+  }
+
+  const initialTask = manager.getTask(taskID)
+  if (initialTask?.sessionID) {
+    return initialTask.sessionID
+  }
+  if (isTerminalStatus(initialTask?.status)) {
+    return undefined
+  }
+
+  const deadline = Date.now() + timeoutMs
+
+  while (Date.now() < deadline) {
+    if (options.signal?.aborted) {
+      return undefined
+    }
+
+    await waitForInterval(intervalMs)
+
+    const task = manager.getTask(taskID)
+    if (task?.sessionID) {
+      return task.sessionID
+    }
+    if (isTerminalStatus(task?.status)) {
+      return undefined
+    }
+  }
+
+  return undefined
+}