CI: no macOS on PRs or main; macOS is built and tested by releases (#22)

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

@@ -23,7 +23,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-15]
+        # No macOS on PRs or main: 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]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4