Why: close the task card inside its own PR so landing it needs no post-merge backfill or separate close PR. Evidence: .task/fix-guard-test-blindspot-coverage.md -> .task/archive/fix-guard-test-blindspot-coverage.md; status 已完成(合入前闭合); PR #671 https://git-home.u1680832.nyat.app:30319/weichen/Agent-System-Dev/pulls/671. Impact: task metadata only; the card is archived within PR #671 and lands with its merge commit. Refs: task.sh close --self