Register the L1 cleanup summary card with task metadata

Why: task-sync/evidence tooling locates a task card by its branch metadata line; the summary-only card was invisible to the card lookup and its sync record could not be recorded.
Evidence: card header added (R0 f979170, standalone, accepting_requirements=false); task-sync rerun after this commit writes the sync receipt into the card.
Impact: Task card bookkeeping only.
Refs: Dev PR #650

.task/final-cleanup-summary.md

@@ -1,3 +1,14 @@
+# L1 最终收尾汇总 任务卡
+
+任务主记录：Dev PR #650（本文件随 PR 描述维护）；状态：待合入；owner：weichen。
+
+## 基线与授权
+
+- 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）= f97917032d143188bc7faf665aff612d9814af4e
+- 工作区：/tmp/wt-l1cleanup（worktree，合入后回收）；分支：feature/l1-final-cleanup-summary
+- 分支类型：standalone；root_target：main；immediate_target：main；accepting_requirements：false
+- 授权与交付终点：L1 IAM 计划 r3 的最终收尾记录——迁移验证快照、Documents #117 规格同步改钉、U7C 闭合说明、状态快照卡；合入 main 即终点。
+
 # L1 IAM Plan Revision 3 最终收尾
 
 **日期**: 2026-10-19 (commit reference 2026-10-19T15:30 UTC)