CI: no macOS on PRs or main; macOS is built and tested by releases (#1820)

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 <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

.github/workflows/ci.yml

@@ -30,9 +30,10 @@ concurrency:
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
-  # Which OSes the test job runs on. main, manual runs and PRs labelled `full-ci` get all three.
-  # Other PRs get Linux, plus macOS and Windows when they touch code whose behaviour differs by
+  # Which OSes the test job runs on. main, manual runs and PRs labelled `full-ci` get Linux and
+  # Windows. Other PRs get Linux, plus Windows when they touch code whose behaviour differs by
   # platform: the app shell, UI, text and fonts, GPU, pen input, build scripts or the lockfile.
+  # macOS is tested only by release.yml.
   platforms:
     name: select platforms
     runs-on: ubuntu-latest
@@ -48,7 +49,9 @@ jobs:
           BASE: ${{ github.event.pull_request.base.sha }}
           FULL: ${{ contains(github.event.pull_request.labels.*.name, 'full-ci') }}
         run: |
-          all='["ubuntu-latest","macos-latest","windows-latest"]'
+          # No macOS on PRs or main: the org's 5 macOS slots are kept for release builds, and
+          # release.yml builds and tests macOS (a failing release is fixed and rebuilt).
+          all='["ubuntu-latest","windows-latest"]'
           if [ "$EVENT" != pull_request ] || [ "$FULL" = true ]; then
             echo "os=$all" >> "$GITHUB_OUTPUT"
             exit 0