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");