feat(gateway): scope systemd service name to HERMES_HOME

Multiple Hermes installations on the same machine now get unique
systemd service names:
- Default ~/.hermes → hermes-gateway (backward compatible)
- Custom HERMES_HOME → hermes-gateway-<8-char-hash>

Changes:
- Add get_service_name() in hermes_cli/gateway.py that derives a
  deterministic service name from HERMES_HOME via SHA256
- Replace all hardcoded 'hermes-gateway' systemd references with
  get_service_name() across gateway.py, main.py, status.py, uninstall.py
- Add HERMES_HOME env var to both user and system systemd unit templates
  so the gateway process uses the correct installation
- Update tests to use get_service_name() in assertions

hermes_cli/status.py

@@ -275,8 +275,13 @@ def show_status(args):
     print(color("◆ Gateway Service", Colors.CYAN, Colors.BOLD))
     
     if sys.platform.startswith('linux'):
+        try:
+            from hermes_cli.gateway import get_service_name
+            _gw_svc = get_service_name()
+        except Exception:
+            _gw_svc = "hermes-gateway"
         result = subprocess.run(
-            ["systemctl", "--user", "is-active", "hermes-gateway"],
+            ["systemctl", "--user", "is-active", _gw_svc],
             capture_output=True,
             text=True
         )