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/external-sources.json

@@ -279,6 +279,10 @@
       "pluginName": "shadcn",
       "category": "Developer Tools",
       "skillPath": "skills/shadcn",
+      "sparseCheckout": [
+        "skills/shadcn",
+        "LICENSE.md"
+      ],
       "brandColor": "#09090B",
       "manifestOverrides": {
         "version": "0.1.0",