fix(cli): print the update-available notice once, not on every event-loop drain

`process.on("beforeExit", ...)` re-fires every time the event loop
drains, and the handler kicks off a fire-and-forget async telemetry
flush — so on a successful command the user sees the
"Update available: …" notice twice (once after the initial drain, again
after the flush settles). Using `process.once` detaches the listener
after first invocation, fixing the double-print and also preventing a
double-flush of telemetry.

Reported during local testing of `auth login`, but the bug affects every
command (any path where `_flush()` schedules work).

packages/cli/src/cli.ts

@@ -193,8 +193,12 @@ if (!isHelp && !hasJsonFlag && command !== "upgrade") {
 const commandStart = Date.now();
 let commandFailed = false;
 
-// Async flush for normal exit (beforeExit fires when the event loop drains)
-process.on("beforeExit", () => {
+// Async flush for normal exit. `beforeExit` re-fires every time the
+// event loop drains, and the async `_flush()` itself schedules new
+// work — so a plain `on` listener would print the update notice (and
+// re-flush) once per drain (the user-reported double-print). `once`
+// detaches after first invocation, which is what we want for both.
+process.once("beforeExit", () => {
   _flush?.().catch(() => {});
   if (!hasJsonFlag) _printUpdateNotice?.();
 });