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

@@ -172,6 +172,18 @@
       },
       "category": "Developer Tools"
     },
+    {
+      "name": "artemis-case-prep",
+      "source": {
+        "source": "local",
+        "path": "./plugins/codex/plugins/artemis-case-prep"
+      },
+      "policy": {
+        "installation": "AVAILABLE",
+        "authentication": "ON_INSTALL"
+      },
+      "category": "Developer Tools"
+    },
     {
       "name": "caveman",
       "source": {