CI: no macOS on PRs or main; macOS is built and tested by releases (#512)

The org has 5 macOS slots in total (GitHub Team), shared by every repo's CI and
release builds, and PR CI kept them full, so release builds queued behind it.
macOS now runs only in release.yml; a release that fails on macOS is fixed and
rebuilt.

Co-authored-by: Brandon Thomas <shared2@storyteller.ai>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -23,12 +23,14 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        # No macOS on PRs, main or nightly: the org's 5 macOS slots are kept for release builds,
+        # and release.yml builds macOS (a failing release is fixed and rebuilt).
+        os: [ubuntu-latest, windows-latest]
     steps:
       - uses: actions/checkout@v4
 
       # The optional craft-fonts build input (Japanese fonts), so the Japanese font tests run on
-      # Linux; macOS and Windows test the build without it. See craftrules standards/fonts.md.
+      # Linux; Windows tests the build without it. See craftrules standards/fonts.md.
       - name: Check out craft-fonts (Linux)
         if: runner.os == 'Linux'
         uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0