fix(gateway): show friendly error when service is not installed

Instead of an unhelpful CalledProcessError traceback when running
`hermes gateway start/stop/restart` without first installing the service,
check for the unit file and exit with an actionable install hint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hermes_cli/gateway.py

@@ -1971,6 +1971,15 @@ def systemd_uninstall(system: bool = False):
     print(f"✓ {_service_scope_label(system).capitalize()} service uninstalled")
 
 
+def _require_service_installed(action: str, system: bool = False) -> None:
+    unit_path = get_systemd_unit_path(system=system)
+    if not unit_path.exists():
+        scope_flag = " --system" if system else ""
+        print(f"✗ Gateway service is not installed")
+        print(f"  Run: {'sudo ' if system else ''}hermes gateway install{scope_flag}")
+        sys.exit(1)
+
+
 def systemd_start(system: bool = False):
     system = _select_systemd_scope(system)
     if system:
@@ -1980,6 +1989,7 @@ def systemd_start(system: bool = False):
         # reachable (common on fresh RHEL/Debian SSH sessions without linger).
         # Raises UserSystemdUnavailableError with a remediation message.
         _preflight_user_systemd()
+    _require_service_installed("start", system=system)
     refresh_systemd_unit_if_needed(system=system)
     _run_systemctl(["start", get_service_name()], system=system, check=True, timeout=30)
     print(f"✓ {_service_scope_label(system).capitalize()} service started")
@@ -1990,6 +2000,7 @@ def systemd_stop(system: bool = False):
     system = _select_systemd_scope(system)
     if system:
         _require_root_for_system_service("stop")
+    _require_service_installed("stop", system=system)
     _run_systemctl(["stop", get_service_name()], system=system, check=True, timeout=90)
     print(f"✓ {_service_scope_label(system).capitalize()} service stopped")
 
@@ -2001,6 +2012,7 @@ def systemd_restart(system: bool = False):
         _require_root_for_system_service("restart")
     else:
         _preflight_user_systemd()
+    _require_service_installed("restart", system=system)
     refresh_systemd_unit_if_needed(system=system)
     from gateway.status import get_running_pid
 

tests/hermes_cli/test_gateway_service.py

@@ -141,6 +141,27 @@ class TestSystemdServiceRefresh:
         assert ["systemctl", "--user", "daemon-reload"] in calls
 
 
+class TestRequireServiceInstalled:
+    def test_exits_with_install_hint_when_unit_missing(self, tmp_path, monkeypatch, capsys):
+        unit_path = tmp_path / "hermes-gateway.service"
+        monkeypatch.setattr(gateway_cli, "get_systemd_unit_path", lambda system=False: unit_path)
+
+        with pytest.raises(SystemExit) as exc_info:
+            gateway_cli._require_service_installed("start")
+
+        assert exc_info.value.code == 1
+        out = capsys.readouterr().out
+        assert "not installed" in out
+        assert "hermes gateway install" in out
+
+    def test_passes_when_unit_exists(self, tmp_path, monkeypatch):
+        unit_path = tmp_path / "hermes-gateway.service"
+        unit_path.write_text("[Unit]\n", encoding="utf-8")
+        monkeypatch.setattr(gateway_cli, "get_systemd_unit_path", lambda system=False: unit_path)
+
+        gateway_cli._require_service_installed("start")
+
+
 class TestGeneratedSystemdUnits:
     def test_user_unit_avoids_recursive_execstop_and_uses_extended_stop_timeout(self):
         unit = gateway_cli.generate_systemd_unit(system=False)
@@ -521,6 +542,7 @@ class TestGatewaySystemServiceRouting:
         calls = []
 
         monkeypatch.setattr(gateway_cli, "_select_systemd_scope", lambda system=False: False)
+        monkeypatch.setattr(gateway_cli, "_require_service_installed", lambda action, system=False: None)
         monkeypatch.setattr(gateway_cli, "refresh_systemd_unit_if_needed", lambda system=False: calls.append(("refresh", system)))
         monkeypatch.setattr(
             "gateway.status.get_running_pid",
@@ -575,6 +597,7 @@ class TestGatewaySystemServiceRouting:
 
     def test_systemd_restart_recovers_failed_planned_restart(self, monkeypatch, capsys):
         monkeypatch.setattr(gateway_cli, "_select_systemd_scope", lambda system=False: False)
+        monkeypatch.setattr(gateway_cli, "_require_service_installed", lambda action, system=False: None)
         monkeypatch.setattr(gateway_cli, "refresh_systemd_unit_if_needed", lambda system=False: None)
         monkeypatch.setattr(
             "gateway.status.read_runtime_status",