feat(.claude-plugin): add graphql-lsp to marketplace (#93)

Register graphql-lsp plugin in marketplace.json to make it
discoverable in the marketplace.

Author: amondnet

.claude-plugin/marketplace.json

@@ -74,6 +74,11 @@
       "description": "Go Language Server",
       "source": "./plugins/gopls-lsp"
     },
+    {
+      "name": "graphql-lsp",
+      "description": "GraphQL Language Server",
+      "source": "./plugins/graphql-lsp"
+    },
     {
       "name": "jdtls-lsp",
       "description": "Java (Eclipse JDTLS) Language Server",