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/collect-validation-errors-with-errorbehavior.good.al

@@ -0,0 +1,37 @@
+codeunit 50185 "Collect Errors Good Sample"
+{
+    [ErrorBehavior(ErrorBehavior::Collect)]
+    procedure ValidateAllItems()
+    var
+        Item: Record Item;
+        CollectedErrors: List of [ErrorInfo];
+        CollectedError: ErrorInfo;
+        ErrorText: Text;
+    begin
+        if Item.FindSet() then
+            repeat
+                // Run each item in its own context so one failure does not abandon the rest.
+                Codeunit.Run(Codeunit::"Collect Errors Item Check", Item);
+            until Item.Next() = 0;
+
+        if HasCollectedErrors() then begin
+            CollectedErrors := GetCollectedErrors();
+            foreach CollectedError in CollectedErrors do
+                ErrorText += CollectedError.Message() + '\';
+            Message('The following must be fixed before posting:\%1', ErrorText);
+        end;
+    end;
+}
+
+codeunit 50186 "Collect Errors Item Check"
+{
+    TableNo = Item;
+
+    trigger OnRun()
+    begin
+        if Rec.Description = '' then
+            Error('Item %1 has no description.', Rec."No.");
+        if Rec."Unit Cost" <= 0 then
+            Error('Item %1 must have a positive unit cost.', Rec."No.");
+    end;
+}