Add error-handling knowledge domain and review leaf skill

Seed the first new AL knowledge domain toward issue #34: a fully wired
error-handling domain that the review pipeline actually consumes.

- 3 knowledge articles (+ good/bad AL samples):
  - prefer-errorinfo-for-actionable-errors (ErrorInfo Fix-it/Show-it actions)
  - collect-validation-errors-with-errorbehavior (ErrorBehavior::Collect)
  - errortype-internal-vs-client-for-diagnostics (ErrorType Internal vs Client)
- New leaf skill al-error-handling-review.md, modeled on al-performance-review
- Wire the leaf into al-code-review (sub-skills, Source, description)
- README: six -> seven leaf skills

Validated: frontmatter validator clean; knowledge-index check passes
(deterministic, full coverage, selection inputs intact).

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

microsoft/knowledge/error-handling/prefer-errorinfo-for-actionable-errors.good.al

@@ -0,0 +1,44 @@
+table 50180 "Actionable Error Good Sample"
+{
+    fields
+    {
+        field(1; "Entry No."; Integer) { }
+        field(2; "Qty. to Invoice"; Decimal)
+        {
+            trigger OnValidate()
+            var
+                CannotInvoiceErr: ErrorInfo;
+            begin
+                if "Qty. to Invoice" > MaxQtyToInvoice() then begin
+                    CannotInvoiceErr.Title := 'Qty. to Invoice isn''t valid';
+                    CannotInvoiceErr.Message := StrSubstNo('You cannot invoice more than %1 units.', MaxQtyToInvoice());
+                    CannotInvoiceErr.DetailedMessage := 'Reduce the quantity to invoice, or apply the maximum allowed.';
+                    CannotInvoiceErr.RecordId := Rec.RecordId();
+                    CannotInvoiceErr.AddAction(
+                        StrSubstNo('Set value to %1', MaxQtyToInvoice()),
+                        Codeunit::"Actionable Error Fixit Sample",
+                        'SetQtyToMax');
+                    Error(CannotInvoiceErr);
+                end;
+            end;
+        }
+    }
+
+    local procedure MaxQtyToInvoice(): Decimal
+    begin
+        exit(10);
+    end;
+}
+
+codeunit 50181 "Actionable Error Fixit Sample"
+{
+    procedure SetQtyToMax(SourceError: ErrorInfo)
+    var
+        Line: Record "Actionable Error Good Sample";
+    begin
+        if Line.Get(SourceError.RecordId) then begin
+            Line.Validate("Qty. to Invoice", 10);
+            Line.Modify(true);
+        end;
+    end;
+}