ci(release): swap retired macos-13 runner for macos-15-intel (#9)

The macos-13 GitHub-hosted runner image was deprecated 2025-09-22 and
fully unsupported from 2025-12-04 (actions/runner-images#13046,
#13402, #13634). Any matrix entry pinning `runner: macos-13` (or
`runs-on: macos-13`) now queues forever — there is no runner.

Replacement is macos-15-intel: the current Intel x64 standard runner
per the actions/runner-images README, stable GA image. Avoided
macos-26-intel (still stabilising — had an arm-label regression in
runner-images#14112 as recently as 2026-05-19) and -large SKUs
(unwanted cost for OSS).

Estate-wide sweep precedent: hyperpolymath/affinescript#292 (merged
today, sha 26cdb43) made the same one-line swap on that repo's
release.yml.  This is the same fix, propagated.

Refs hyperpolymath/affinescript#292, actions/runner-images#13046.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath