fix(skill): add YAML frontmatter to SKILL.md so skills CLI can detect it

The `skills` CLI (https://github.com/openclaw/skills) requires a YAML
frontmatter block with `name` and `description` to recognize a SKILL.md
as a valid skill. The current file declares description as a Markdown
blockquote, which causes:

  $ npx skills add jackwener/wx-cli -g
  No valid skills found. Skills require a SKILL.md with name and description.

Switching to standard frontmatter makes installation work end-to-end.

Verified with `npx skills add . -l`:
  Found 1 skill
    wx-cli

SKILL.md

@@ -1,6 +1,9 @@
-# wx-cli
+---
+name: wx-cli
+description: "wx-cli — 从本地微信数据库查询聊天记录、联系人、会话、收藏等。用户提到微信聊天记录、联系人、消息历史、群成员、收藏内容时，使用此 skill 安装并调用 wx-cli。"
+---
 
-> description: "wx-cli — 从本地微信数据库查询聊天记录、联系人、会话、收藏等。用户提到微信聊天记录、联系人、消息历史、群成员、收藏内容时，使用此 skill 安装并调用 wx-cli。"
+# wx-cli
 
 ## Triggers