Stop tracking .mcp.json; it carries a real API key

.mcp.json needs NOVELLY_API_KEY to match the API's Auth:ServiceApiKey
user secret, so it can't be a checked-in file — gitignore it and keep
.mcp.json.example (with the key blanked out) as the template. README
walks through copying the example and setting the matching user secret.

.mcp.json.example

@@ -3,7 +3,8 @@
     "novelly": {
       "command": "./mcp-server/Novelly.Mcp",
       "env": {
-        "NOVELLY_API_URL": "http://localhost:5080"
+        "NOVELLY_API_URL": "http://localhost:5080",
+        "NOVELLY_API_KEY": "<matches the API's Auth:ServiceApiKey user secret>"
       }
     }
   }