Fold-close the task card before merge

Why: close the task card inside its own PR so landing it needs no post-merge backfill or separate close PR.
Evidence: .task/feature-fast-slow-o2-a01-p5-record.md -> .task/archive/feature-fast-slow-o2-a01-p5-record.md; status 已完成（合入前闭合）; PR #673 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/673.
Impact: task metadata only; the card is archived within PR #673 and lands with its merge commit.
Refs: task.sh close --self

.task/archive/feature-fast-slow-o2-a01-p5-record.md

@@ -1,6 +1,6 @@
 # feature/fast-slow-o2-a01-p5-record 任务卡
 
-任务主记录：PR #673 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/673；状态：进行中；owner：weichen
+任务主记录：PR #673 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/673；状态：已完成（合入前闭合）；owner：weichen
 
 ## 基线与授权
 
@@ -153,3 +153,9 @@
   "receipt_path": "/home/weichen/.cache/agent-system-dev/evidence/34ba043bf5679b760ccc206a92dc2b4776b1480d372fff4ad698ab228a308761.json"
 }
 ```
+
+## 闭合记录（合入前折叠）
+
+- 本卡在 PR #673（https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/673）合入前闭合并归档，随该 PR 一同合入。
+- 该 PR 的合并提交即本任务的落地记录。
+- 合入后无需回填，也无需单独的闭合 PR。