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

tests/hermes_cli/test_gateway_linger.py

@@ -114,7 +114,7 @@ def test_systemd_install_calls_linger_helper(monkeypatch, tmp_path, capsys):
     assert unit_path.exists()
     assert [cmd for cmd, _ in calls] == [
         ["systemctl", "--user", "daemon-reload"],
-        ["systemctl", "--user", "enable", gateway.SERVICE_NAME],
+        ["systemctl", "--user", "enable", gateway.get_service_name()],
     ]
     assert helper_calls == [True]
     assert "User service installed and enabled" in out