fix: SKILL.md os 字段 macos 改为 darwin，修复 macOS 上技能被 blocked

OpenClaw 使用 Node.js process.platform 匹配平台，macOS 对应值为
darwin 而非 macos，导致所有 6 个技能在 macOS 上被标记为
Missing requirements 无法使用。

Closes #21

@@ -13,7 +13,7 @@ metadata:
     emoji: "\U0001F4D5"
     homepage: https://github.com/xpzouying/xiaohongshu-skills
     os:
-      - macos
+      - darwin
       - linux
 ---
 

@@ -12,7 +12,7 @@ metadata:
         - uv
     emoji: "\U0001F510"
     os:
-      - macos
+      - darwin
       - linux
 ---
 

@@ -12,7 +12,7 @@ metadata:
         - uv
     emoji: "\U0001F4CA"
     os:
-      - macos
+      - darwin
       - linux
 ---
 

@@ -12,7 +12,7 @@ metadata:
         - uv
     emoji: "\U0001F50D"
     os:
-      - macos
+      - darwin
       - linux
 ---
 

@@ -12,7 +12,7 @@ metadata:
         - uv
     emoji: "\U0001F4AC"
     os:
-      - macos
+      - darwin
       - linux
 ---
 

@@ -12,7 +12,7 @@ metadata:
         - uv
     emoji: "\U0001F4DD"
     os:
-      - macos
+      - darwin
       - linux
 ---