Backfill the merge and cleanup records in the archived U6 card

Why: Close automation filled the header (PR #408) but merge/cleanup
lines still described the in-progress state.

Evidence: CI green on PR head (one 405 race auto-resynced);
task-finish recycled at e80baf1 on 2026-10-12.

Impact: Archived card text only.

Refs: PR #408; PR #409

.task/archive/feature-generative-ui-u6.md

@@ -82,8 +82,8 @@
 
 ## 合入与清理
 
-- 合入记录：进行中——Backend #163、Frontend #62 已合入；Dev 发布 PR 见主记录行；CI 于 PR HEAD 为权威验收。
-- 清理记录：NOT_RUN（授权依据、写者退出确认、worktree/分支/任务卡处置）
+- 合入记录：已合入——Backend #163、Frontend #62、Dev PR #408（CI ci/test 通过，一次 405 竞态补同步）。
+- 清理记录：已完成——TASK_WRITERS_STOPPED=1 make task-finish 于 2026-10-12 回收（observed_head e80baf1）；闭合经 PR #409。
 
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