fix(gateway): don't dead-end setup wizard when only system-scope unit is installed

The setup wizard dropped non-root users at a bare shell prompt when
trying to start a system-scope gateway service. Previously
_require_root_for_system_service called sys.exit(1), which the
wizard's `except Exception` guards cannot catch (SystemExit is a
BaseException). Users with a pre-existing /etc/systemd/system unit
(e.g. from an earlier `sudo hermes setup` run) hit this whenever
they re-ran `hermes setup` as a regular user.

- Convert _require_root_for_system_service to raise a typed
  SystemScopeRequiresRootError (RuntimeError subclass) instead of
  sys.exit(1). The direct CLI path (`hermes gateway install|start|stop|
  restart|uninstall` without sudo) still exits 1 cleanly via a new
  catch at the top of gateway_command, matching the existing
  UserSystemdUnavailableError pattern.
- Add _system_scope_wizard_would_need_root() pre-check and
  _print_system_scope_remediation() helper. Both setup wizards
  (hermes_cli/setup.py and hermes_cli/gateway.py::gateway_setup) now
  detect the dead-end before prompting and print actionable guidance:
  either `sudo systemctl start <service>` this time, or uninstall the
  system unit and install a per-user one.
- Defense-in-depth: all 5 wizard prompt sites also catch
  SystemScopeRequiresRootError and fall back to the remediation
  helper if the pre-check is bypassed (race, etc.).

Tests: 12 new tests in TestSystemScopeRequiresRootError,
TestSystemScopeWizardPreCheck, TestSystemScopeRemediationOutput, and
TestGatewayCommandCatchesSystemScopeError covering the exception
contract, pre-check matrix (root vs non-root, system-only vs
user-present vs none vs explicit system=True), remediation output
for each action, and the direct-CLI exit-1 path.

hermes_cli/setup.py

@@ -2462,6 +2462,9 @@ def setup_gateway(config: dict):
             launchd_start,
             launchd_restart,
             UserSystemdUnavailableError,
+            SystemScopeRequiresRootError,
+            _system_scope_wizard_would_need_root,
+            _print_system_scope_remediation,
         )
 
         service_installed = _is_service_installed()
@@ -2479,7 +2482,9 @@ def setup_gateway(config: dict):
             print()
 
         if service_running:
-            if prompt_yes_no("  Restart the gateway to pick up changes?", True):
+            if supports_systemd and _system_scope_wizard_would_need_root():
+                _print_system_scope_remediation("restart")
+            elif prompt_yes_no("  Restart the gateway to pick up changes?", True):
                 try:
                     if supports_systemd:
                         systemd_restart()
@@ -2489,10 +2494,19 @@ def setup_gateway(config: dict):
                     print_error("  Restart failed — user systemd not reachable:")
                     for line in str(e).splitlines():
                         print(f"  {line}")
+                except SystemScopeRequiresRootError as e:
+                    # Defense in depth: the pre-check above should have
+                    # caught this, but a race (unit file appearing mid-run)
+                    # could still land here. Previously this exited the
+                    # whole wizard via sys.exit(1).
+                    print_error(f"  Restart failed: {e}")
+                    _print_system_scope_remediation("restart")
                 except Exception as e:
                     print_error(f"  Restart failed: {e}")
         elif service_installed:
-            if prompt_yes_no("  Start the gateway service?", True):
+            if supports_systemd and _system_scope_wizard_would_need_root():
+                _print_system_scope_remediation("start")
+            elif prompt_yes_no("  Start the gateway service?", True):
                 try:
                     if supports_systemd:
                         systemd_start()
@@ -2502,6 +2516,9 @@ def setup_gateway(config: dict):
                     print_error("  Start failed — user systemd not reachable:")
                     for line in str(e).splitlines():
                         print(f"  {line}")
+                except SystemScopeRequiresRootError as e:
+                    print_error(f"  Start failed: {e}")
+                    _print_system_scope_remediation("start")
                 except Exception as e:
                     print_error(f"  Start failed: {e}")
         elif supports_service_manager:
@@ -2529,6 +2546,9 @@ def setup_gateway(config: dict):
                             print_error("  Start failed — user systemd not reachable:")
                             for line in str(e).splitlines():
                                 print(f"  {line}")
+                        except SystemScopeRequiresRootError as e:
+                            print_error(f"  Start failed: {e}")
+                            _print_system_scope_remediation("start")
                         except Exception as e:
                             print_error(f"  Start failed: {e}")
                 except Exception as e: