Sharpen the sync evidence warning to non-card changes only

Why: evidence rows are written before their containing card commit, so a naive
HEAD-diff would flag every task's evidence as stale and train people to ignore
the warning.

Evidence: bash -n clean; the live task-sync for this task exercises the new
path (warning fires only when a non-.task diff follows an evidence row's HEAD).

Impact: scripts/task.sh cmd_sync now excludes .task from the staleness diff;
AGENTS.md wording aligned (three-output evidence via make evidence).

Refs: task card evidence table; guidelines verification.md tool-written rule.

scripts/task.sh

@@ -189,14 +189,16 @@ cmd_sync() {
     printf '| 同步前验收 | %s | %s | %s |\n' "$base" "$head" "$(date -u +%FT%TZ)" >> "$card"
     git add "$card"
     git commit -q -m "Record the sync checkpoint" || true
-    # [evidence] 同步改变了被测对象：卡片证据表中旧 HEAD 的行自动失效，关键检查须重跑。
-    local eh stale_ev="" short_head
+    # [evidence] 同步改变了被测对象：卡片证据表中旧 HEAD 的行，其后若有非 .task 变更则失效须重跑。
+    local eh stale_ev="" short_head changed
     short_head=$(echo "$head" | cut -c1-10)
     while IFS='|' read -r _ _ _ eh _ _ _; do
       eh=$(echo "$eh" | tr -d ' `')
-      [ -n "$eh" ] && [ "$eh" != "$short_head" ] && stale_ev="$stale_ev $eh"
+      { [ -n "$eh" ] && [ "$eh" != "$short_head" ]; } || continue
+      changed=$(git diff --name-only "${eh}" HEAD -- . ':(exclude).task' 2>/dev/null | head -1)
+      [ -n "$changed" ] && stale_ev="$stale_ev ${eh}"
     done < <(grep -E '^\| [0-9]{4}-[0-9]{2}-[0-9]{2}T[^|]*\|' "$card" 2>/dev/null || true)
-    [ -n "$stale_ev" ] && echo "[evidence] WARN 证据表含旧被测 HEAD:$(echo $stale_ev | tr ' ' '\n' | sort -u | tr '\n' ' ')（≠ 当前 $short_head）——同步后关键检查须重跑并以 make evidence 重新写入"
+    [ -n "$stale_ev" ] && echo "[evidence] WARN 证据表被测 HEAD 落后且其后有非卡片变更:$(echo $stale_ev | tr ' ' '\n' | sort -u | tr '\n' ' ')（当前 $short_head）——关键检查须重跑并以 make evidence 重新写入"
   else
     echo "[warn] 找不到分支 ${branch} 的任务卡，同步记录未写入" >&2
   fi