Make marketplace onboarding formal for Chinese users

Add a market-source-adapter plugin so maintainers have a reusable workflow for adapting third-party skill repositories while replacing the old test-oriented README with official Chinese marketplace guidance.

Constraint: Public plugins must remain installable as independent Codex plugins and Chinese-facing fields must be localized without changing stable plugin IDs.

Rejected: Keep adapter guidance only in README | Maintainers need it as an installable skill available from the same marketplace and KeyInfo skill generation page.

Confidence: high

Scope-risk: narrow

Directive: Add future third-party repository structures as explicit adapters or source config, not one-off copy steps.

Tested: python scripts/validate_marketplace.py; python /Users/eapil/.codex/skills/.system/plugin-creator/scripts/validate_plugin.py plugins/codex/plugins/market-source-adapter; python /Users/eapil/.codex/skills/.system/skill-creator/scripts/quick_validate.py backend/data/skill_market_repo/plugins/codex/plugins/market-source-adapter/skills/market-source-adapter; node local skills.json visibility check

Not-tested: Installing market-source-adapter through Codex Desktop UI.

plugins/codex/README.md

@@ -1,12 +1,24 @@
-# Codex Marketplace Root
+# Codex 插件市场目录
 
-This directory can be used as a local marketplace root:
+这个目录是 EAPIL Codex 插件市场的稀疏安装入口。通常推荐从仓库根目录添加市场，并把稀疏路径留空；只有在需要减少拉取范围时才使用本目录。
+
+从仓库根目录安装市场：
+
+```powershell
+codex plugin marketplace add https://git.playones.com/arechen/EapilSkillMarket.git --ref main
+```
+
+使用稀疏路径安装市场：
+
+```powershell
+codex plugin marketplace add https://git.playones.com/arechen/EapilSkillMarket.git --ref main --sparse .agents --sparse plugins/codex
+```
+
+本地调试：
 
 ```powershell
 codex plugin marketplace add .\plugins\codex
 codex plugin add docx@eapil-skill-market
 ```
 
-For Git marketplace testing from the repository root, prefer the root `.agents/plugins/marketplace.json` entry and leave sparse checkout empty.
-
-Each skill directory is packaged as a separate plugin so users can install only what they need.
+每个 `plugins/<plugin-name>` 目录都是独立插件，用户可以只安装自己需要的插件。插件内可以包含一个或多个相关 Skill。