test(acl): OpenZaak default-fills ZGW fields and posts to OpenZaak (refs #5)

Add the ACL skeleton (Application/Infrastructure/Api per §9) and failing
xUnit tests for the single operation: AclService.OpenZaakAsync must
default-fill bronorganisatie, verantwoordelijkeOrganisatie, startdatum
(clock) and vertrouwelijkheidaanduiding and delegate to the gateway; the
OpenZaakGateway must POST the zaak to OpenZaak with a Bearer JWT and the
default-filled body. Implementations throw NotImplementedException — red.

Also add ADR-0003 (default-fill strategy).

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

services/acl/Acl.Infrastructure/SystemClock.cs

@@ -0,0 +1,8 @@
+using Acl.Application;
+
+namespace Acl.Infrastructure;
+
+public sealed class SystemClock : IClock
+{
+    public DateOnly Today => DateOnly.FromDateTime(DateTime.UtcNow);
+}