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/.agents/plugins/marketplace.json

@@ -148,6 +148,18 @@
       },
       "category": "开发工具"
     },
+    {
+      "name": "market-source-adapter",
+      "source": {
+        "source": "local",
+        "path": "./plugins/market-source-adapter"
+      },
+      "policy": {
+        "installation": "AVAILABLE",
+        "authentication": "ON_INSTALL"
+      },
+      "category": "开发工具"
+    },
     {
       "name": "eapil-gpt2-image",
       "source": {