#7187: fix(gateway): suppress AbortError during shutdown

## Summary Suppress `AbortError` during gateway shutdown to prevent noisy crash logs. **Problem**: When the gateway shuts down, in-flight HTTP requests throw `AbortError` which propagates as unhandled rejection — fills logs with false-alarm crash reports. **Fix**: Catch `AbortError` specifically during shutdown and log at debug level instead of crashing. ## Risk Summary - **Severity**: Low (log noise, not a functional issue) - **Regression risk**: None — only suppresses errors during shutdown path, normal operation unchanged - **Scope**: Single file, +32 lines ## Verification - [x] `pnpm build` passes - [x] `pnpm lint` passes - [x] `pnpm test` passes ## Sign-Off - Models used: Claude Opus 4.6 - Submitter effort: Traced shutdown signal flow through gateway-daemon, verified AbortError detection uses error.name check (works across Node versions), confirmed debug-level log preserves observability - Agent notes: N/A lobster-biscuit