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-agent.service.j2

@@ -6,6 +6,7 @@ Wants=network-online.target
 
 [Service]
 Type=simple
+WorkingDirectory=/opt/decnet
 Environment=DECNET_SYSTEM_LOGS=/var/log/decnet/decnet.agent.log
 ExecStart=/usr/local/bin/decnet agent --no-forwarder
 Restart=on-failure