Add interfaces knowledge domain and review leaf skill (#42)

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: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

microsoft/knowledge/interfaces/assign-codeunit-to-interface-for-testability.good.al

@@ -0,0 +1,51 @@
+interface IDiscountCalculation
+{
+    procedure CalculateDiscount(Amount: Decimal): Decimal;
+}
+
+codeunit 50213 "Standard Discount Calc" implements IDiscountCalculation
+{
+    procedure CalculateDiscount(Amount: Decimal): Decimal
+    begin
+        // Production rule: 10% off amounts over 1000.
+        if Amount > 1000 then
+            exit(Amount * 0.1);
+        exit(0);
+    end;
+}
+
+codeunit 50214 "Test Discount Calc" implements IDiscountCalculation
+{
+    // Lightweight test double: a fixed, predictable value so a test can assert
+    // order totals without depending on the production discount rule.
+    procedure CalculateDiscount(Amount: Decimal): Decimal
+    begin
+        exit(100);
+    end;
+}
+
+codeunit 50215 "Order Total"
+{
+    var
+        DiscountCalc: Interface IDiscountCalculation;
+
+    // Production wiring: a codeunit assigns directly to the interface variable.
+    procedure UseProductionCalculation()
+    var
+        StdCalc: Codeunit "Standard Discount Calc";
+    begin
+        DiscountCalc := StdCalc;
+    end;
+
+    // Setter injection: a test passes "Test Discount Calc" instead, with no
+    // enum and no change to the consumer. The dependency is an interface.
+    procedure SetDiscountCalculation(NewDiscountCalc: Interface IDiscountCalculation)
+    begin
+        DiscountCalc := NewDiscountCalc;
+    end;
+
+    procedure NetAmount(Amount: Decimal): Decimal
+    begin
+        exit(Amount - DiscountCalc.CalculateDiscount(Amount));
+    end;
+}