chore(skills): drop dead SAFETY_PREAMBLE_INJECT marker

No injection code ever existed in this repository, so the marker was
inert in all 22 multi SKILL.md files. Remove it and correct the skill
authoring docs to describe safety rules as plain prose.

Co-authored-by: Cursor <cursoragent@cursor.com>

skills/multi/dingtalk-chat/SKILL.md

@@ -16,8 +16,6 @@ metadata:
 
 > **PREREQUISITE:** Read the `dws-shared` skill first for auth, global flags, product routing, URL preflight, error codes, and safety rules. The `dws` binary must be on PATH.
 
-<!-- SAFETY_PREAMBLE_INJECT -->
-
 > ⚠️ **命令可用性以当前 dws 二进制为准**。服务发现已下线，本文档随内置 skill 发布；如果 `dws <cmd> --help` 不存在，说明当前版本未暴露该命令。若命令存在但调用失败，请按错误中的 endpoint 或 tool 提示确认静态端点目录和后端工具注册。实际调用前可用 `dws <cmd> --help` 或 `--dry-run` 验证。