docs: add PR body notes

pr-body.md

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+## Summary
+Add a JSON config knob to control unhandled promise rejection behavior in the Gateway/CLI.
+
+## Motivation
+Transient network errors (e.g., undici `fetch failed`) can currently terminate the gateway when they surface as unhandled promise rejections. Some operators prefer warn-only behavior to avoid missed replies and restarts.
+
+## Changes
+- Add `gateway.unhandledRejections: "warn"|"exit"` to config types + zod schema.
+- Wire config into `installUnhandledRejectionHandler({ mode })` in `src/index.ts` and `src/cli/run-main.ts`.
+- Extend handler to accept `mode` and skip `process.exit(1)` when `mode="warn"`.
+
+## Default behavior
+Unchanged (defaults to `"exit"`).
+
+## Acceptance criteria
+- With `gateway.unhandledRejections="warn"`, unhandled rejections are logged but do not terminate the gateway.
+- With default config, behavior remains unchanged.