fix: resolve systemd restart loop with --replace flag (#576)

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

hermes_cli/main.py

@@ -1315,6 +1315,8 @@ For more help on a command:
     # gateway run (default)
     gateway_run = gateway_subparsers.add_parser("run", help="Run gateway in foreground")
     gateway_run.add_argument("-v", "--verbose", action="store_true")
+    gateway_run.add_argument("--replace", action="store_true",
+                             help="Replace any existing gateway instance (useful for systemd)")
     
     # gateway start
     gateway_start = gateway_subparsers.add_parser("start", help="Start gateway service")