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/assign-codeunit-to-interface-for-testability.bad.al

@@ -0,0 +1,22 @@
+codeunit 50217 "Standard Discount Calc Bad"
+{
+    procedure CalculateDiscount(Amount: Decimal): Decimal
+    begin
+        if Amount > 1000 then
+            exit(Amount * 0.1);
+        exit(0);
+    end;
+}
+
+codeunit 50216 "Order Total Bad"
+{
+    // Anti-pattern: the dependency is a concrete codeunit type, so a test
+    // cannot substitute a double - it always runs the production rule.
+    var
+        DiscountCalc: Codeunit "Standard Discount Calc Bad";
+
+    procedure NetAmount(Amount: Decimal): Decimal
+    begin
+        exit(Amount - DiscountCalc.CalculateDiscount(Amount));
+    end;
+}