fix(ci): add setup-node 22 for Object.groupBy support

eslint-flat-config-utils v3.1.0 (dep of @antfu/eslint-config v8) uses
Object.groupBy which requires Node.js 21+. The CI runner defaulted to
an older Node.js version.

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

Author: amondnet

.github/workflows/ci.yml

@@ -12,6 +12,10 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
+        with:
+          node-version: 22
+
       - uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
         with:
           bun-version: latest