feat(acl): implement OpenZaak default-fill + ZGW gateway (refs #5)

AclService.OpenZaakAsync builds a ZaakRequest from the configured defaults
(bronorganisatie, verantwoordelijkeOrganisatie, vertrouwelijkheidaanduiding,
zaaktype) plus today's date from the clock, and delegates to the gateway.
OpenZaakGateway POSTs the zaak to /zaken/api/v1/zaken with a ZGW Bearer JWT
and the CRS headers, returning the created zaak URL. Tests pass (green).

Add a root register-referentie.slnx (bff + acl) and point `make ci` at it so
the gate covers every .NET service.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/acl/Acl.Application/AclService.cs

@@ -5,5 +5,16 @@ namespace Acl.Application;
 public sealed class AclService(IZaakGateway gateway, AclDefaults defaults, IClock clock)
 {
     public Task<Uri> OpenZaakAsync(DomainRegistration registration, CancellationToken ct = default)
-        => throw new NotImplementedException();
+    {
+        ArgumentNullException.ThrowIfNull(registration);
+
+        var request = new ZaakRequest(
+            defaults.Bronorganisatie,
+            defaults.VerantwoordelijkeOrganisatie,
+            defaults.Vertrouwelijkheidaanduiding,
+            defaults.ZaaktypeUrl,
+            clock.Today);
+
+        return gateway.OpenZaakAsync(request, ct);
+    }
 }