Add breaking-changes knowledge domain and review leaf skill

Seed a new breaking-changes (AL API stability) knowledge domain with six
articles plus good/bad AL samples, a new al-breaking-changes-review leaf
skill, and minimal wiring into al-code-review and the README.

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

microsoft/knowledge/breaking-changes/choose-access-modifiers-deliberately.bad.al

@@ -0,0 +1,21 @@
+codeunit 50326 "Order Processor Bad"
+{
+    // Anti-pattern: every helper is public by default, exposing implementation
+    // detail as a de-facto API. Each becomes a contract that cannot be changed
+    // without risking breakage for consumers that bound to it.
+    procedure ProcessOrder(OrderNo: Code[20])
+    begin
+        ValidateOrder(OrderNo);
+        PostOrder(OrderNo);
+    end;
+
+    procedure ValidateOrder(OrderNo: Code[20])
+    begin
+        if OrderNo = '' then
+            Error('Order number is required.');
+    end;
+
+    procedure PostOrder(OrderNo: Code[20])
+    begin
+    end;
+}