fix(gateway): scrub _HERMES_GATEWAY from POSIX detached restart watcher too

Follow-up to the salvaged #41264 (Windows watcher): the setsid/bash detached
restart watcher on Linux/macOS inherits _HERMES_GATEWAY=1 the same way, so
the CLI's self-restart loop guard silently refuses 'hermes gateway restart'
and the gateway never comes back. Scrub the marker from the watcher env on
the POSIX branch as well, and extend the setsid test to assert it.

gateway/run.py

@@ -4338,12 +4338,20 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
             f"while kill -0 {current_pid} 2>/dev/null; do sleep 0.2; done; "
             f"{cmd} gateway restart"
         )
+        # Same marker scrub as the Windows watcher above: this watcher runs
+        # `hermes gateway restart` from outside the gateway, but it inherits
+        # _HERMES_GATEWAY=1 from us, and the CLI's self-restart loop guard
+        # refuses to run when that marker is set — silently (DEVNULL), so the
+        # gateway stops and never comes back.
+        watcher_env = os.environ.copy()
+        watcher_env.pop("_HERMES_GATEWAY", None)
         setsid_bin = shutil.which("setsid")
         if setsid_bin:
             subprocess.Popen(
                 [setsid_bin, "bash", "-lc", shell_cmd],
                 stdout=subprocess.DEVNULL,
                 stderr=subprocess.DEVNULL,
+                env=watcher_env,
                 start_new_session=True,
             )
         else:
@@ -4351,6 +4359,7 @@ class GatewayRunner(GatewayAuthorizationMixin, GatewayKanbanWatchersMixin, Gatew
                 ["bash", "-lc", shell_cmd],
                 stdout=subprocess.DEVNULL,
                 stderr=subprocess.DEVNULL,
+                env=watcher_env,
                 start_new_session=True,
             )