Add interfaces knowledge domain and review leaf skill

Adds the interfaces knowledge domain covering AL interfaces and enum-with-implementation: three atomic articles with good/bad AL samples, a new al-interfaces-review leaf skill, and additive wiring into al-code-review and the README. Purely additive; no contract change.

Part of #34.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

microsoft/knowledge/interfaces/prefer-interface-over-case-branching.good.al

@@ -0,0 +1,47 @@
+interface IShippingRate
+{
+    procedure CalculateRate(Weight: Decimal): Decimal;
+}
+
+codeunit 50200 "Standard Shipping Rate" implements IShippingRate
+{
+    procedure CalculateRate(Weight: Decimal): Decimal
+    begin
+        exit(Weight * 1.5);
+    end;
+}
+
+codeunit 50201 "Express Shipping Rate" implements IShippingRate
+{
+    procedure CalculateRate(Weight: Decimal): Decimal
+    begin
+        exit((Weight * 1.5) + 25);
+    end;
+}
+
+enum 50202 "Shipping Method" implements IShippingRate
+{
+    Extensible = true;
+
+    value(0; Standard)
+    {
+        Implementation = IShippingRate = "Standard Shipping Rate";
+    }
+    value(1; Express)
+    {
+        Implementation = IShippingRate = "Express Shipping Rate";
+    }
+}
+
+codeunit 50203 "Shipping Charge"
+{
+    // Dispatch is automatic: assign the enum to the interface variable and call.
+    // A new method = one new enum value + one impl codeunit, with no edit here.
+    procedure GetRate(Method: Enum "Shipping Method"; Weight: Decimal): Decimal
+    var
+        RateProvider: Interface IShippingRate;
+    begin
+        RateProvider := Method;
+        exit(RateProvider.CalculateRate(Weight));
+    end;
+}