ci: replace deprecated macos-13 runner with macos-14/macos-15

GitHub has deprecated macos-13 (x86_64) runners. Use macos-14 and
macos-15 (both arm64 Apple Silicon) for CI builds. Release workflow
now produces a single macOS ARM64 binary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: steinbeck

.github/workflows/build.yml

@@ -57,10 +57,10 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: macos-13
-            arch: x86_64
           - os: macos-14
             arch: arm64
+          - os: macos-15
+            arch: arm64-m15
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4

.github/workflows/release.yml

@@ -13,9 +13,6 @@ jobs:
           - os: ubuntu-latest
             artifact: surge-linux-x86_64
             binary: surge
-          - os: macos-13
-            artifact: surge-macos-x86_64
-            binary: surge
           - os: macos-14
             artifact: surge-macos-arm64
             binary: surge
@@ -133,7 +130,6 @@ jobs:
 
             ### Binaries
             - `surge-linux-x86_64.tar.gz` - Linux (x86_64)
-            - `surge-macos-x86_64.tar.gz` - macOS (Intel)
             - `surge-macos-arm64.tar.gz` - macOS (Apple Silicon)
             - `surge-windows-x86_64.tar.gz` - Windows (x86_64, built with MSYS2)
           files: |