Use GitHub-hosted macOS 26 runners

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
 
   test-macos:
     name: Test (macOS)
-    runs-on: ghcr.io/cirruslabs/macos-runner:sequoia
+    runs-on: macos-26
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v6

.github/workflows/release.yml

@@ -13,7 +13,7 @@ jobs:
   release:
     name: Release
     if: github.ref_type == 'tag'
-    runs-on: ghcr.io/cirruslabs/macos-runner:sequoia
+    runs-on: macos-26
     timeout-minutes: 60
     environment: publish
     steps:
@@ -56,7 +56,7 @@ jobs:
   dry-run:
     name: Release (Dry Run)
     if: github.ref_type != 'tag'
-    runs-on: ghcr.io/cirruslabs/macos-runner:sequoia
+    runs-on: macos-26
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@v6