When running under systemd, the gateway could enter restart loops in two
scenarios:
1. The previous gateway process hasn't fully exited when systemd starts
a new one, causing 'Gateway already running (PID ...)' → exit 1 →
restart → same error → infinite loop.
2. The interactive CLI exits immediately in non-TTY mode, and systemd
keeps restarting it.
Changes:
- Add --replace flag to 'hermes gateway run' that gracefully kills any
existing gateway instance (SIGTERM → wait 10s → SIGKILL) before
starting, preventing the PID-lock deadlock.
- Update the generated systemd unit template to use --replace by default,
add ExecStop for clean shutdown, set KillMode=mixed and
TimeoutStopSec=15 for proper process management.
- Existing behavior (without --replace) is unchanged: still prints the
error message and exits, now also mentioning the --replace option.
Fixes#576
ee5daba061