feat(config): add swarmctl-host to INI, env, CLI; drop hardcoded bind from systemd unit

[swarm] swarmctl-host → DECNET_SWARMCTL_HOST so operators set the bind
address once in decnet.ini; `decnet swarmctl` and the systemd unit both
resolve it via envvar — no --host/--port pinned on ExecStart.

decnet/cli/init.py

@@ -74,6 +74,7 @@ _CONFIG_PLACEHOLDER = """\
 # master-host = 10.0.0.1
 # syslog-port = 6514
 # swarmctl-port = 8770
+# swarmctl-host = 127.0.0.1
 
 # [logging]
 # system-log = /var/log/decnet/decnet.system.log