fix(scripts): bump-objectui.sh --help 用哨兵定界，改 header 不再静默截断 (#6425) (#6470)

`--help` 过去用硬编码行号 `sed -n '2,26p'` 打印自己的头注释，行号与 header 内容之间
没有任何机制耦合：往 header 加一行帮助就静默截断，删一行就越界打进下一段。两种情况都
exit 0、都打印了「一些东西」，所以截断的帮助和完整的帮助长得一模一样。

这不是假设。#5960 往 header 加了 pin 更新步骤，把真正的结尾从第 19 行推到第 26 行，
PR #6421 只能手工挪这个魔法数字，并留下一条「请下一位作者注意」的 NOTE。注释不是机制。

改法是把行号换成哨兵 `# --help ends here`，让终止符与它所终止的内容放在一起：

- `sed -n '2,/^# --help ends here$/p'` 取到哨兵为止。起始的 `2` 定位的是 shebang，
  位置由 execve 固定，不随 header 内容漂移，因此不是会漂的行号。
- `grep -vxF` 按整行精确剔掉哨兵本身，而不是按子串过滤，header 里即便提到这句话也不会
  被连带吃掉。
- 哨兵缺失时 exit 1 并说明如何恢复，而不是一路打印到 EOF（实测无此闸会吐出 231 行）。
  闸只设在 --help 分支：删掉一条注释不该让真正的 pin bump 跑不起来。

同时删掉 `:66-68` 那条来自 PR #6421 的手工耦合 NOTE —— 哨兵落地后它即成假话，留着
只是把谎言搬个家。

验证：改动前后 `--help` 输出逐字节一致（25 行 / 1363 字节）；往 header 加一行后新行
出现，而同一探针在旧脚本上被静默丢弃。


Claude-Session: https://claude.ai/code/session_01BDmDsu2575gDxeMCxXhDE3

Co-authored-by: Claude <noreply@anthropic.com>

Author: hotlong

scripts/bump-objectui.sh

@@ -24,6 +24,13 @@
 # Assumes sibling layout:
 #   ~/work/objectui
 #   ~/work/objectstack   ← run from here
+# --help ends here
+#
+# ^ SENTINEL, not prose — `--help` prints from the shebang down to the line above
+# and stops there, so the terminator travels with the text it terminates. Add or
+# remove header lines freely; no line number tracks this block any more (#6425).
+# Spell it exactly: the --help branch below refuses to run without it. Everything
+# from here down is internal rationale and is NOT user-facing help.
 #
 # objectui ships @object-ui/console as a static SPA. The framework
 # release pipeline reads .objectui-sha, clones objectui at that commit,
@@ -63,10 +70,25 @@ for arg in "$@"; do
     --no-commit) NO_COMMIT=1 ;;
     --no-changeset) NO_CHANGESET=1 ;;
     -h|--help)
-      # NOTE: this line range is coupled to the header block above (usage → env →
-      # sibling layout, ending at "run from here"). Editing the header means moving
-      # it — #5960 added the pin-update step and had to.
-      sed -n '2,26p' "$0" | sed 's/^# \{0,1\}//'
+      # The header block above IS the help text, and the `# --help ends here`
+      # sentinel is what ends it — no line range, so growing the header can no
+      # longer truncate the help (#6425; #5960 grew it and PR #6421 had to move a
+      # hand-kept `2,26p`). The leading `2` addresses the shebang, whose position
+      # is fixed by execve rather than by the header's content, so it cannot drift.
+      #
+      # A missing sentinel EXITS 1 rather than running on to EOF: a truncated help
+      # and a complete one both exit 0 and both print something, which is precisely
+      # why the old coupling could fail in silence — same lesson as the `head -40`
+      # this script used to truncate its changeset list with (#4731). Guarded here,
+      # not at startup: a deleted comment must never stop an actual pin bump.
+      if ! grep -qxF '# --help ends here' "$0"; then
+        echo "✗ ${0##*/}: the '# --help ends here' sentinel is missing — cannot tell" >&2
+        echo "  where the help text ends. Restore it at the end of the header block." >&2
+        exit 1
+      fi
+      sed -n '2,/^# --help ends here$/p' "$0" \
+        | grep -vxF '# --help ends here' \
+        | sed 's/^# \{0,1\}//'
       exit 0
       ;;
     *) EXPLICIT_SHA="$arg" ;;