Add comment explaining coverage restriction

Author: jeffsmale90

shared/config/vitest.config.ts

@@ -1,5 +1,7 @@
 import { defineConfig } from 'vitest/config';
 
+// @vitest/coverage-v8 imports `node:inspector/promises`, which Node 18 does not
+// provide. Coverage is enabled on Node 20+ only so CI can still run tests on 18.x.
 const coverageEnabled = Number(process.versions.node.split('.')[0]) >= 20;
 
 export default defineConfig({