feat(skills): make Codex the default install target

SETUP.md

@@ -1,4 +1,6 @@
-# Setup Guide
+# Claude Code Marketplace Setup
+
+本页只描述 Claude Code marketplace。Codex 用户直接运行 `./install-skills.sh`，技能默认安装到 `~/.agents/skills`。
 
 ## 1. Clone the Gitea Repository
 
@@ -114,7 +116,7 @@ ai-proj-helper/
 - Check plugin name is correct
 - Ensure marketplace.json is valid: `cat .claude-plugin/marketplace.json | jq`
 
-**"Skills not working"**
+**"Skills not working in Claude Code"**
 - Skills are Agent Skills (auto-invoked by Claude when relevant)
 - They don't create slash commands
 - Check plugin installation: `/plugin list`