Link the U3 contract task card to its PR

Why: the card and PR description are one record.

Evidence: PR #115 created via the Gitea API.

Impact: task card only.

Refs: Documents PR #115
Co-authored-by: Cursor <cursoragent@cursor.com>

.task/feature-iam-u3-account-contract.md

@@ -1,7 +1,7 @@
 # feature/iam-u3-account-contract 任务卡
 
 任务主记录：本文件随 PR 描述维护。
-状态：已完成（合入前闭合）；owner：weichen。PR：待创建
+状态：已完成（合入前闭合）；owner：weichen。PR：https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Documents/pulls/115
 
 ## 基线与授权