fix: use node:timers/promises for async setTimeout

node:util does not export setTimeout — the async sleep function
lives in node:timers/promises.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/plugins/runtime-tmux/src/index.ts

@@ -1,5 +1,6 @@
 import { execFile } from "node:child_process";
-import { promisify, setTimeout as sleep } from "node:util";
+import { promisify } from "node:util";
+import { setTimeout as sleep } from "node:timers/promises";
 import { randomUUID } from "node:crypto";
 import { writeFileSync, unlinkSync } from "node:fs";
 import { tmpdir } from "node:os";