ci: add Node.js 26 to the test matrix (#141)

Stacked on #140, so the diff here is just the workflow change.

Node 26 is the current release line (26.6.0, released 2026-08-03) and
becomes LTS in October. Running it now surfaces breakage before the
promotion rather than after.

`engines` stays at `>= 18`, so this only widens what CI covers.

Author: fengmk2

.github/workflows/nodejs.yml

@@ -15,6 +15,6 @@ jobs:
     uses: node-modules/github-actions/.github/workflows/node-test.yml@master
     with:
       os: 'ubuntu-latest, macos-latest, windows-latest'
-      version: '18, 20, 22, 24'
+      version: '18, 20, 22, 24, 26'
     secrets:
       CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}