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.bad.al

@@ -0,0 +1,21 @@
+table 50182 "Actionable Error Bad Sample"
+{
+    fields
+    {
+        field(1; "Entry No."; Integer) { }
+        field(2; "Qty. to Invoice"; Decimal)
+        {
+            trigger OnValidate()
+            begin
+                // Dead-end error: the code knows the maximum but offers the user no way to apply it.
+                if "Qty. to Invoice" > MaxQtyToInvoice() then
+                    Error('You cannot invoice more than %1 units.', MaxQtyToInvoice());
+            end;
+        }
+    }
+
+    local procedure MaxQtyToInvoice(): Decimal
+    begin
+        exit(10);
+    end;
+}