fix(core): default null task.description to empty string on persist

The tasks table description column is NOT NULL, so a task arriving with
description == null/undefined would fail the insert with a constraint
error. Default to "" in getTaskPersistValues, matching the ?? null / ?? 0
treatment of other optional fields.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/store.ts

@@ -1743,7 +1743,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       task.id,
       task.lineageId ?? generateTaskLineageId(),
       task.title ?? null,
-      task.description,
+      task.description ?? "",
       normalizeTaskPriority(task.priority),
       task.column,
       task.status ?? null,