fix: raise default maxListeners to suppress benign gz stream warning

[BYK]

Summary

• Raises events.defaultMaxListeners from 10 to 15 in packages/gateway/instrument.ts to eliminate a benign MaxListenersExceededWarning that fires once per OpenCode session.

Root Cause

When the gateway runs in-process inside OpenCode (Bun), upstream fetch() calls receive gzip-compressed responses. Bun's internal fetch creates a zlib.Gunzip transform stream, and the Web Streams adapter + OpenCode's Effect-TS runtime attach 11 listeners to it — 1 above the default limit of 10.

Logs confirm this is not a leak: exactly 1 warning per process (4 occurrences across 4 PIDs over a week), always exactly 11 listeners, never growing. All listeners are cleaned up when the response body is consumed.

Fix

Two lines at the top of instrument.ts:

import { setMaxListeners } from "node:events";
setMaxListeners(15);

15 is still low enough to catch real leaks (which grow to hundreds).