docs: note child process bridge helper

docs/gateway/background-process.md

@@ -26,6 +26,10 @@ Behavior:
 - Output is kept in memory until the session is polled or cleared.
 - If the `process` tool is disallowed, `exec` runs synchronously and ignores `yieldMs`/`background`.
 
+## Child process bridging
+
+When spawning long-running child processes outside the exec/process tools (for example, CLI respawns or gateway helpers), attach the child-process bridge helper so termination signals are forwarded and listeners are detached on exit/error. This avoids orphaned processes on systemd and keeps shutdown behavior consistent across platforms.
+
 Environment overrides:
 - `PI_BASH_YIELD_MS`: default yield (ms)
 - `PI_BASH_MAX_OUTPUT_CHARS`: in‑memory output cap (chars)