ci: pin macOS runner to macos-26

Match the local dev environment (ARM Mac on macOS 26) instead of
relying on the macos-latest label, which lags behind real releases.

Author: mhiro2

.github/workflows/ci.yaml

@@ -26,7 +26,7 @@ jobs:
     strategy:
       matrix:
         os:
-          - macos-latest
+          - macos-26
           - ubuntu-latest
     steps:
       - name: Checkout repository