feat(KB-087): remove Created/Updated columns and clean up usage tracking

- Remove createdAt and updatedAt columns from ListView component
- Update TaskCard to remove date display and related styles
- Update TaskDetailModal to remove date fields
- Update ListView tests to reflect new column layout
- Fix column index in dependencies test after column removal
- Remove unused usage tracking module (usage.ts and usage.test.ts)
- Clean up related CSS styles for date columns

packages/engine/src/executor.ts

@@ -526,7 +526,7 @@ export class TaskExecutor {
         }
         executorLog.error(`✗ ${task.id} execution failed:`, err.message);
         await this.store.logEntry(task.id, `Execution failed: ${err.message}`);
-        await this.store.updateTask(task.id, { status: "failed", error: err.message });
+        await this.store.updateTask(task.id, { status: "failed" });
         this.options.onError?.(task, err);
       }
     } finally {