feat(enroll): systemd units for agent/forwarder/engine + log-directory INI key

Rename log-file-path -> log-directory (maps to DECNET_LOG_DIRECTORY). Bundle
now ships three systemd units rendered with agent_name/master_host and installs
them into /etc/systemd/system/. Bootstrap replaces direct 'decnet X --daemon'
calls with systemctl enable --now. Each unit pins DECNET_SYSTEM_LOGS so agent,
forwarder, and deckies logs land at decnet.{agent,forwarder}.log and decnet.log
under /var/log/decnet.

decnet/web/templates/decnet-engine.service.j2

@@ -0,0 +1,16 @@
+[Unit]
+Description=DECNET deckie orchestrator (decnet deploy) — {{ agent_name }}
+Documentation=https://github.com/anti/DECNET
+After=network-online.target decnet-agent.service
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+Environment=DECNET_SYSTEM_LOGS=/var/log/decnet/decnet.log
+ExecStart=/usr/local/bin/decnet deploy
+StandardOutput=append:/var/log/decnet/decnet.log
+StandardError=append:/var/log/decnet/decnet.log
+
+[Install]
+WantedBy=multi-user.target