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.

config/assets/artemis-case-prep.svg

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128" role="img" aria-labelledby="title">
+  <title id="title">ARTEMIS case prep</title>
+  <rect x="8" y="8" width="112" height="112" rx="25" fill="#177C80"/>
+  <path d="M36 22h43l17 17v61c0 4-3 7-7 7H36c-4 0-7-3-7-7V29c0-4 3-7 7-7Z" fill="#F8FBFA"/>
+  <path d="M79 22v13c0 3 3 6 6 6h11Z" fill="#A6E6D6"/>
+  <rect x="40" y="44" width="26" height="8" rx="4" fill="#FFBF56"/>
+  <rect x="40" y="61" width="34" height="6" rx="3" fill="#B8D7D8"/>
+  <rect x="40" y="74" width="24" height="6" rx="3" fill="#B8D7D8"/>
+  <circle cx="82" cy="80" r="22" fill="#FF6C68"/>
+  <path d="m72 80 7 7 13-15" fill="none" stroke="#FFFFFF" stroke-width="7" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>