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.

tests/test_deployer.py

@@ -43,7 +43,7 @@ class TestCompose:
         _compose("up", "-d", compose_file=Path("test.yml"))
         mock_run.assert_called_once()
         cmd = mock_run.call_args[0][0]
-        assert cmd[:4] == ["docker", "compose", "-f", "test.yml"]
+        assert cmd[:6] == ["docker", "compose", "-p", "decnet", "-f", "test.yml"]
         assert "up" in cmd
         assert "-d" in cmd