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-web.service

@@ -1,27 +1,35 @@
 [Unit]
 Description=DECNET Web Dashboard Service
-After=network.target decnet-api.service
+Documentation=https://github.com/4nt11/DECNET/wiki/Web-Dashboard
+After=network-online.target decnet-api.service
+Wants=network-online.target
 
 [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 web command
-ExecStart=/path/to/DECNET/.venv/bin/decnet web
+WorkingDirectory=/opt/decnet
+EnvironmentFile=-/opt/decnet/.env.local
+ExecStart=/opt/decnet/venv/bin/decnet web
 
-# The Web Dashboard service does not require network administration privileges.
-# Enable the following lines if you wish to bind the Dashboard to a privileged port (e.g., 80 or 443) 
-# while still running as a non-root user.
+# Uncomment if you bind the dashboard to a privileged port (80/443):
 # CapabilityBoundingSet=CAP_NET_BIND_SERVICE
 # AmbientCapabilities=CAP_NET_BIND_SERVICE
+CapabilityBoundingSet=
+AmbientCapabilities=
 
 # Security Hardening
 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
+ReadOnlyPaths=/etc/decnet
 
 Restart=on-failure
 RestartSec=5