Merge pull request #1164 from stan-dev/test-on-mac26

add macos-26 to CI runs

Author: jgabry

.github/workflows/R-CMD-check.yaml

@@ -35,6 +35,7 @@ jobs:
         config:
           - {os: macOS-latest, r: 'devel', opencl: true}
           - {os: macOS-latest, r: 'release', opencl: true}
+          - {os: macos-26, r: 'release', opencl: false}
           - {os: macos-15-intel, r: 'release', opencl: false}
           - {os: windows-latest, r: 'devel' }
           - {os: windows-latest, r: 'release'}