Add task-card autoclose wiring: close subcommand, post-merge hook, gate reports

Why: 11 merged branches had left their task cards open (bookkeeping debt found on 2026-09-30); closure was manual and never triggered, so card status rotted after each merge.
Evidence: dogfood run of `task.sh close --fold` closed and archived the 11 cards (previous commit); temp-repo matrix 33/33 PASS covering detection, pending-branch idempotence, dirty/no-fold refusals, post-merge auto-close on merge-type pulls, worktree guard, and the full lifecycle (closure PR merge archives cards on main).
Impact: scripts/task.sh (+close, precheck [cards] report, finish-tail trigger), scripts/hooks/post-merge (new), Makefile (+task-close target, setup chmod), AGENTS.md rule; archive fixes correct PR links missed by the first dogfood run; no module or gitlink change.
Refs: task card feature/card-autoclose-r1

scripts/hooks/post-merge

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+# 本地钩子（post-merge）：merge 型 pull/merge 落地后自动闭合已合入任务卡（ff-only 不触发
+# 本钩子——由 task-finish 收尾与 precheck [cards] 报告兜底）。
+# 触发条件收紧到"主检出 + 集成分支 main"——任务 worktree 里 task-sync 的 merge 不触发，
+# 防止把卡片闭合混进任务分支现场。失败只警告，绝不阻断 pull。
+# 注意：task.sh close 在主检出会另建 feature/card-close-* 分支提交（集成分支不直推），
+# 结束后切回 main；PR 由所有者合入。
+common=$(git rev-parse --git-common-dir) && common=$(cd "$common" 2>/dev/null && pwd) || exit 0
+mainrepo=$(dirname "$common")
+[ "$mainrepo" = "$PWD" ] || exit 0
+[ "$(git rev-parse --abbrev-ref HEAD)" = "main" ] || exit 0
+[ -f "$mainrepo/scripts/task.sh" ] || exit 0
+bash "$mainrepo/scripts/task.sh" close \
+  || echo "[warn] post-merge 卡片自动闭合失败——在主检出手动运行 make task-close" >&2
+exit 0