feat(deploy): systemd units w/ capability-based hardening; updater restarts agent via systemctl

Add deploy/ unit files for every DECNET daemon (agent, updater, api, web,
swarmctl, listener, forwarder). All run as User=decnet with NoNewPrivileges,
ProtectSystem, PrivateTmp, LockPersonality; AmbientCapabilities=CAP_NET_ADMIN
CAP_NET_RAW only on the agent (MACVLAN/scapy). Existing api/web units migrated
to /opt/decnet layout and the same hardening stanza.

Make the updater's _spawn_agent systemd-aware: under systemd (detected via
INVOCATION_ID + systemctl on PATH), `systemctl restart decnet-agent.service`
replaces the Popen path so the new agent inherits the unit's ambient caps
instead of the updater's empty set. _stop_agent becomes a no-op in that mode
to avoid racing systemctl's own stop phase.

Tests cover the dispatcher branch selection, MainPID parsing, and the
systemd no-op stop.

deploy/decnet-agent.service

@@ -0,0 +1,41 @@
+[Unit]
+Description=DECNET Worker Agent (mTLS)
+Documentation=https://github.com/4nt11/DECNET/wiki/SWARM-Mode
+After=network-online.target docker.service
+Wants=network-online.target
+Requires=docker.service
+
+[Service]
+Type=simple
+User=decnet
+Group=decnet
+# docker.sock is group-readable by 'docker'; the agent needs it for compose.
+SupplementaryGroups=docker
+WorkingDirectory=/opt/decnet
+EnvironmentFile=-/opt/decnet/.env.local
+ExecStart=/opt/decnet/venv/bin/decnet agent --host 0.0.0.0 --port 8765 --agent-dir /etc/decnet/agent
+
+# MACVLAN/IPVLAN management + scapy raw sockets. Granted via ambient caps so
+# the process starts unprivileged and keeps only these two bits.
+CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_RAW
+AmbientCapabilities=CAP_NET_ADMIN CAP_NET_RAW
+
+# Security Hardening
+NoNewPrivileges=yes
+ProtectSystem=full
+ProtectHome=read-only
+PrivateTmp=yes
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectControlGroups=yes
+RestrictSUIDSGID=yes
+LockPersonality=yes
+# /opt/decnet holds release slots + state; the agent reads them and writes its PID.
+ReadWritePaths=/opt/decnet /var/log/decnet
+
+Restart=on-failure
+RestartSec=5
+TimeoutStopSec=15
+
+[Install]
+WantedBy=multi-user.target