Guard task-start against reusing a merged branch name

Why: 合入后再 task-start 同名分支会从新 main 长出第二段历史并覆盖已合入任务卡
（2026-10-05 EXT-1 复核轮实测，两轮证据差点丢失）；原校验只看本地分支能否创建。
Evidence: bash -n 通过；功能负例——start feature/ext-1-inproc-handler 被拒且无
worktree 副作用；make test 全链 exit 0；task-sync 基线 43d33aa。
Impact: 仅 scripts/task.sh cmd_start 增加已合入同名分支检查，其余子命令行为不变。
Refs: EXT-1 任务卡复核轮记录（.task/archive/feature-ext-1-inproc-handler.md）。

scripts/task.sh

@@ -102,9 +102,18 @@ cmd_precheck() {
 
 cmd_start() {
   need_repo
-  local name="${1:-}"; [ -n "$name" ] || die "usage: task.sh start <feature|fix|hotfix>/<slug>"
+  local name="${1:-}"; [ -n "$name" ] || die "usage: task.sh start <feature|fix|hotfix|release>/<slug>"
   echo "$name" | grep -qE '^(feature|fix|hotfix|release)/[A-Za-z0-9._-]+$' || die "branch name must match (feature|fix|hotfix|release)/<slug>"
   git fetch -q origin "${TARGET_BRANCH}" 2>/dev/null || true
+  # 已合入的同名分支不得重建（2026-10-05 实测：合入后再 task-start 同名会从新 main 长出
+  # 第二段历史，空卡覆盖已合入任务卡，两轮证据差点丢失）。复核/追加沿用原分支或换新 slug。
+  local guard_ref
+  for guard_ref in "refs/heads/${name}" "refs/remotes/origin/${name}"; do
+    if git rev-parse -q --verify "${guard_ref}" >/dev/null \
+      && git merge-base --is-ancestor "${guard_ref}" "origin/${TARGET_BRANCH}"; then
+      die "branch ${name} already merged into origin/${TARGET_BRANCH} — reuse the original branch or pick a new slug (task-start must not recreate merged cards)"
+    fi
+  done
   [ "$(git status --porcelain | { grep -v '^.. \.zcode/' || true; } | wc -l)" -eq 0 ] || die "dirty tree — resolve first (precheck verdict BUSY)"
   [ "$(git submodule status --recursive 2>/dev/null | { grep -cE '^\+|^U' || true; })" -eq 0 ] || die "submodule drift/conflict — resolve first (precheck verdict BUSY)"
   git merge-base --is-ancestor "origin/${TARGET_BRANCH}" HEAD || die "local ${TARGET_BRANCH} behind origin — pull --ff-only first"