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-final-budget.md -> .task/archive/feature-fast-slow-o2-a01-final-budget.md; status 已完成（合入前闭合）; PR #672 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/672.
Impact: task metadata only; the card is archived within PR #672 and lands with its merge commit.
Refs: task.sh close --self

.task/archive/feature-fast-slow-o2-a01-final-budget.md

@@ -1,6 +1,6 @@
 # feature/fast-slow-o2-a01-final-budget 任务卡
 
-任务主记录：PR #672 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/672；状态：进行中；owner：weichen
+任务主记录：PR #672 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/672；状态：已完成（合入前闭合）；owner：weichen
 
 ## 基线与授权
 
@@ -158,3 +158,9 @@
   "receipt_path": "/home/weichen/.cache/agent-system-dev/evidence/9471a0b4da4b6690df9e11984a620cec535b1873025dbf393a73e7ad26e029e7.json"
 }
 ```
+
+## 闭合记录（合入前折叠）
+
+- 本卡在 PR #672（https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/672）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。