feat(KB-082): complete Step 2 — Record failure error in executor

packages/core/src/types.ts

@@ -148,6 +148,8 @@ export interface Task {
   validatorModelId?: string;
   /** Number of merge retry attempts made for this task (auto-merge conflict recovery) */
   mergeRetries?: number;
+  /** Error message from the last failure, if task status is 'failed' */
+  error?: string;
   /** ISO-8601 timestamp of when the task last entered its current column.
    *  Used to sort cards within a column so that recently-moved cards appear at the top. */
   columnMovedAt?: string;