fix(engine): pin docker compose project name to avoid empty-basename failure

systemd daemons run with WorkingDirectory=/ by default; docker compose
derives the project name from basename(cwd), which is empty at '/', and
aborts with 'project name must not be empty'. Pass -p decnet explicitly
so the project name is independent of cwd, and set WorkingDirectory=/opt/decnet
on the three DECNET units so compose artifacts (decnet-compose.yml,
build contexts) also land in the install dir.

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

@@ -7,6 +7,7 @@ PartOf=decnet-agent.service
 
 [Service]
 Type=simple
+WorkingDirectory=/opt/decnet
 Environment=DECNET_SYSTEM_LOGS=/var/log/decnet/decnet.forwarder.log
 ExecStart=/usr/local/bin/decnet forwarder --master-host {{ master_host }} --master-port 6514 --agent-dir /etc/decnet/agent --log-file /var/log/decnet/decnet.log
 Restart=on-failure