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-i2-backend.md -> .task/archive/feature-fast-slow-o2-i2-backend.md; status 已完成(合入前闭合); PR #652 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/652. Impact: task metadata only; the card is archived within PR #652 and lands with its merge commit. Refs: task.sh close --self