Update GitHub Actions CI to use Node.js 20.x and 22.x

Drop Node.js 18.x support due to compatibility issues with Vitest 4.

The error occurred because:
- Vitest 4 with coverage uses node:inspector/promises
- This module was introduced in Node.js 19.0.0
- Node.js 18.x doesn't have this module, causing CI failures

Updated matrix to use:
- Node.js 20.x (current LTS)
- Node.js 22.x (latest stable)

This ensures compatibility with Vitest 4 and modern tooling.

Author: claude

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [18.x, 20.x]
+        node-version: [20.x, 22.x]
 
     steps:
     - name: Checkout code