fix: use "./" instead of "." for plugin source path in marketplace.json

The Claude Code marketplace schema validation rejects "." as a plugin
source value. Using "./" fixes the "Invalid input" error when running
`claude plugin marketplace add greensock/gsap-skills`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -13,7 +13,7 @@
       "author": {
         "name": "GreenSock"
       },
-      "source": ".",
+      "source": "./",
       "category": "productivity"
     }
   ]