On fresh RHEL/Debian SSH sessions without linger, `systemctl --user
start hermes-gateway` fails with 'Failed to connect to bus: No medium
found' because /run/user/$UID/bus doesn't exist. Setup previously
showed a raw CalledProcessError and continued claiming success, so the
gateway never actually started.
systemd_start() and systemd_restart() now call _preflight_user_systemd()
for the user scope first:
- Bus socket already there → no-op (desktop / linger-enabled servers)
- Linger off → try loginctl enable-linger (works when polkit permits,
needs sudo otherwise), wait for socket
- Still unreachable → raise UserSystemdUnavailableError with a clean
remediation message pointing to sudo loginctl + hermes gateway run
as the foreground fallback
Setup's start/restart handlers and gateway_command() catch the new
exception and render the multi-line guidance instead of a traceback.
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