fix(gateway): use setsid instead of systemd-run --user for /update (salvage #4024) (#4104)

Salvaged from PR #4024 by @Sertug17. Fixes #4017.

- Replace systemd-run --user --scope with setsid for portable session detach
- Add system-level service detection to cmd_update gateway restart
- Falls back to start_new_session=True on systems without setsid (macOS, minimal containers)

gateway/run.py

@@ -4617,8 +4617,8 @@ class GatewayRunner:
     async def _handle_update_command(self, event: MessageEvent) -> str:
         """Handle /update command — update Hermes Agent to the latest version.
 
-        Spawns ``hermes update`` in a separate systemd scope so it survives the
-        gateway restart that ``hermes update`` may trigger at the end. Marker
+        Spawns ``hermes update`` in a detached session (via ``setsid``) so it
+        survives the gateway restart that ``hermes update`` may trigger. Marker
         files are written so either the current gateway process or the next one
         can notify the user when the update finishes.
         """
@@ -4658,28 +4658,28 @@ class GatewayRunner:
         pending_path.write_text(json.dumps(pending))
         exit_code_path.unlink(missing_ok=True)
 
-        # Spawn `hermes update` in a separate cgroup so it survives gateway
-        # restart. systemd-run --user --scope creates a transient scope unit.
+        # Spawn `hermes update` detached so it survives gateway restart.
+        # Use setsid for portable session detach (works under system services
+        # where systemd-run --user fails due to missing D-Bus session).
         hermes_cmd_str = " ".join(shlex.quote(part) for part in hermes_cmd)
         update_cmd = (
             f"{hermes_cmd_str} update > {shlex.quote(str(output_path))} 2>&1; "
             f"status=$?; printf '%s' \"$status\" > {shlex.quote(str(exit_code_path))}"
         )
         try:
-            systemd_run = shutil.which("systemd-run")
-            if systemd_run:
+            setsid_bin = shutil.which("setsid")
+            if setsid_bin:
+                # Preferred: setsid creates a new session, fully detached
                 subprocess.Popen(
-                    [systemd_run, "--user", "--scope",
-                     "--unit=hermes-update", "--",
-                     "bash", "-c", update_cmd],
+                    [setsid_bin, "bash", "-c", update_cmd],
                     stdout=subprocess.DEVNULL,
                     stderr=subprocess.DEVNULL,
                     start_new_session=True,
                 )
             else:
-                # Fallback: best-effort detach with start_new_session
+                # Fallback: start_new_session=True calls os.setsid() in child
                 subprocess.Popen(
-                    ["bash", "-c", f"nohup {update_cmd} &"],
+                    ["bash", "-c", update_cmd],
                     stdout=subprocess.DEVNULL,
                     stderr=subprocess.DEVNULL,
                     start_new_session=True,