Add Node 20+ version check and engines field

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

package.json

@@ -1,8 +1,11 @@
 {
   "name": "@companion-ai/feynman",
-  "version": "0.2.4",
+  "version": "0.2.5",
   "description": "Research-first CLI agent built on Pi and alphaXiv",
   "type": "module",
+  "engines": {
+    "node": ">=20.18.1"
+  },
   "bin": {
     "feynman": "bin/feynman.js"
   },
@@ -57,9 +60,6 @@
     "tsx": "^4.21.0",
     "typescript": "^5.9.3"
   },
-  "engines": {
-    "node": ">=20.18.1"
-  },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/getcompanion-ai/feynman.git"