Make the canonical package.json private instead of declaring MIT (closes #119)

Repositories copy package.json to get prettier, and with "license": "MIT"
each of them declared MIT whatever its own licence is. "private": true
keeps yarn from printing "No license field" and makes no licence claim.
This repository's LICENSE and README License section are unchanged.

Model: opus-5-5

package.json

@@ -1,5 +1,5 @@
 {
-  "license": "MIT",
+  "private": true,
   "devDependencies": {
     "prettier": "3.8.1"
   }