Fix CI: drop Node 18, require Node 20+ (vitest ESM compat)

Author: joeynyc

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [18, 20, 22]
+        node-version: [20, 22]
 
     steps:
     - name: Checkout code

package.json

@@ -59,6 +59,6 @@
     "vitest": "^4.0.18"
   },
   "engines": {
-    "node": ">=16.0.0"
+    "node": ">=20.0.0"
   }
 }