Close the EXT-20 task card before merge

Why: Card closes in the same PR per delivery rules.
Evidence: Card only.
Impact: None.
Refs: https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Backend/pulls/212
Co-authored-by: Cursor <cursoragent@cursor.com>

.task/feature-regulations-kb-ext20.md

@@ -1,7 +1,7 @@
 # feature/regulations-kb-ext20 任务卡
 
 任务主记录：本文件随 PR 描述维护。
-状态：进行中；owner：weichen。
+状态：已完成（合入前闭合）；owner：weichen。PR：https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Backend/pulls/212
 
 ## 基线与授权