Add Node 20+ version check and engines field

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

bin/feynman.js

@@ -1,2 +1,8 @@
 #!/usr/bin/env node
-import "../dist/index.js";
+const v = process.versions.node.split(".").map(Number);
+if (v[0] < 20) {
+  console.error(`feynman requires Node.js 20 or later (you have ${process.versions.node})`);
+  console.error("upgrade: https://nodejs.org or nvm install 20");
+  process.exit(1);
+}
+import("../dist/index.js");

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@companion-ai/feynman",
-  "version": "0.2.4",
+  "version": "0.2.5",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@companion-ai/feynman",
-      "version": "0.2.4",
+      "version": "0.2.5",
       "hasInstallScript": true,
       "dependencies": {
         "@companion-ai/alpha-hub": "^0.1.2",

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"