fix(ci): drop Node 18 from CI matrix

Vitest/rolldown requires node:util.styleText (Node 20.1+).
Node 18 is EOL, keeping 20 and 22 in the matrix.

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

Author: talionwar

.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:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4