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-updater.service

@@ -0,0 +1,49 @@
+[Unit]
+Description=DECNET Self-Updater (mTLS)
+Documentation=https://github.com/4nt11/DECNET/wiki/Remote-Updates
+After=network-online.target
+Wants=network-online.target
+# Deliberately NOT After=decnet-agent.service — the updater must come up even
+# when the agent is broken, since that is exactly when it is most useful.
+
+[Service]
+Type=simple
+User=decnet
+Group=decnet
+WorkingDirectory=/opt/decnet
+EnvironmentFile=-/opt/decnet/.env.local
+ExecStart=/opt/decnet/venv/bin/decnet updater \
+    --host 0.0.0.0 --port 8766 \
+    --updater-dir /etc/decnet/updater \
+    --install-dir /opt/decnet \
+    --agent-dir /etc/decnet/agent
+
+# The updater SIGTERMs the agent and spawns a new one. Same User=decnet means
+# signalling is allowed without CAP_KILL. It does not need NET_ADMIN/NET_RAW
+# itself — the new agent process picks those up from decnet-agent.service when
+# systemd restarts it (or from the agent's own unit's AmbientCapabilities when
+# spawned by the updater as a direct child).
+CapabilityBoundingSet=
+AmbientCapabilities=
+
+# Security Hardening
+NoNewPrivileges=yes
+ProtectSystem=full
+ProtectHome=read-only
+PrivateTmp=yes
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectControlGroups=yes
+RestrictSUIDSGID=yes
+LockPersonality=yes
+# Writes release slots, pip installs into venv, manages agent.pid.
+ReadWritePaths=/opt/decnet /var/log/decnet
+
+Restart=on-failure
+RestartSec=5
+# Self-update replaces the process image via os.execv; the new binary answers
+# /health within 30 s. Give it headroom before systemd's own termination.
+TimeoutStopSec=30
+
+[Install]
+WantedBy=multi-user.target