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-api.service

@@ -1,19 +1,21 @@
 [Unit]
 Description=DECNET API Service
-After=network.target docker.service
+Documentation=https://github.com/4nt11/DECNET/wiki/REST-API-Reference
+After=network-online.target docker.service
+Wants=network-online.target
 Requires=docker.service
 
 [Service]
 Type=simple
 User=decnet
 Group=decnet
-WorkingDirectory=/path/to/DECNET
-# Ensure environment is loaded from the .env file
-EnvironmentFile=/path/to/DECNET/.env
-# Use the virtualenv python to run the decnet api command
-ExecStart=/path/to/DECNET/.venv/bin/decnet api
+# docker.sock is group-readable by 'docker'; the API ingester tails container logs.
+SupplementaryGroups=docker
+WorkingDirectory=/opt/decnet
+EnvironmentFile=-/opt/decnet/.env.local
+ExecStart=/opt/decnet/venv/bin/decnet api
 
-# Capabilities required to manage MACVLAN interfaces and network links without root
+# MACVLAN/IPVLAN setup runs from the API lifespan when the embedded sniffer is on.
 CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_RAW
 AmbientCapabilities=CAP_NET_ADMIN CAP_NET_RAW
 
@@ -21,9 +23,17 @@ AmbientCapabilities=CAP_NET_ADMIN CAP_NET_RAW
 NoNewPrivileges=yes
 ProtectSystem=full
 ProtectHome=read-only
+PrivateTmp=yes
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectControlGroups=yes
+RestrictSUIDSGID=yes
+LockPersonality=yes
+ReadWritePaths=/opt/decnet /var/log/decnet
 
 Restart=on-failure
 RestartSec=5
+TimeoutStopSec=15
 
 [Install]
 WantedBy=multi-user.target