Sync third-party and MCP marketplace plugins

Constraint: Public skills are published only by explicit administrator action unless they are tracked third-party market sources.
Confidence: high
Scope-risk: narrow
Directive: Keep private/internal skills out of the public marketplace and preserve normal incremental market Git history.
Tested: Marketplace validation passed.

.agents/plugins/marketplace.json

@@ -400,18 +400,6 @@
       },
       "category": "Creativity"
     },
-    {
-      "name": "postgres",
-      "source": {
-        "source": "local",
-        "path": "./plugins/codex/plugins/postgres"
-      },
-      "policy": {
-        "installation": "AVAILABLE",
-        "authentication": "ON_INSTALL"
-      },
-      "category": "Data & Analytics"
-    },
     {
       "name": "literature-review",
       "source": {