Link the W3 remainder card to its pull request

Why: the task card and PR description are one document and must point at each other.

Evidence: PR #641 opened against feature/rdm-two-trees-followup.

Impact: task card only.

Refs: PR #641
Co-authored-by: Cursor <cursoragent@cursor.com>

.task/feature-rdm-two-trees-followup-w3-rest.md

@@ -1,6 +1,6 @@
 # feature/rdm-two-trees-followup-w3-rest 任务卡
 
-任务主记录：<PR 链接，交付前填写>；状态：进行中；owner：weichen
+任务主记录：PR [#641](https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/641)（草稿）；状态：待用户阶段 6 人审；owner：weichen
 
 - 权威仓：ssh://git@git-ssh.u1680832.nyat.app:31411/weichen/Agent-System-Dev.git；目标分支：main；R0（initial_base_sha）= 2b7fdc39a1f44163f4766701429d61ffd6926f1a
 - 工作区：/home/weichen/projects/Agent-System-Dev-wt-feature-rdm-two-trees-followup-w3-rest（worktree）；分支：feature/rdm-two-trees-followup-w3-rest