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.

AGENTS.md

@@ -17,11 +17,10 @@
 - `.agents/plugins/marketplace.json`：仓库根市场入口，适合 Git 市场直接添加。
 - `plugins/codex/.agents/plugins/marketplace.json`：兼容稀疏路径安装的嵌套市场入口。
 
-插件目录：
+插件目录统一放在 `plugins/codex/plugins/<plugin-name>/`。当前市场包含自制公开插件和可同步第三方插件，具体插件列表以两个 marketplace 文件为准：
 
-- `plugins/codex/plugins/docx`
-- `plugins/codex/plugins/superpowers`
-- `plugins/codex/plugins/ui-ux-pro-max`
+- `.agents/plugins/marketplace.json`
+- `plugins/codex/.agents/plugins/marketplace.json`
 
 当前市场只保留正式公开插件。历史测试插件不应继续出现在 marketplace 或 `plugins/codex/plugins/` 下，下一次发布或同步时也应移除未被索引的测试目录，避免用户安装到过期能力。