ci: migrate macOS runner from macos-13 to macos-15-intel

GitHub Actions deprecated macOS 13 (Ventura) runners in September 2025,
with full retirement completed by December 2025. The workflows were
failing because the macos-13 runner image is no longer available.

Migrated to macos-15-intel, which is the new Intel (x86_64) runner
running macOS 15 Sequoia. This ensures continued support for building
universal macOS binaries that work on both Intel and Apple Silicon Macs.

The macos-15-intel runner is supported until August 2027, after which
GitHub Actions will no longer support x86_64 architecture for macOS.
At that point, we'll need to either transition to ARM-only builds or
implement cross-compilation.

Reference: actions/runner-images#13046

Author: secondfry

.github/workflows/main.yml

@@ -2,9 +2,9 @@ name: CI
 
 on:
   push:
-    branches: [ master, develop ]
+    branches: [master, develop]
   pull_request:
-    branches: [ master, develop ]
+    branches: [master, develop]
   workflow_dispatch:
 
 jobs:
@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [windows-latest, macos-13]
+        os: [windows-latest, macos-15-intel]
     steps:
       - uses: actions/checkout@v3
 
@@ -58,17 +58,17 @@ jobs:
           name: shortcircuit.exe
           path: ./dist/shortcircuit.exe
 
-      - name: Additional dependencies [macos-13]
-        if: ${{ matrix.os == 'macos-13' }}
+      - name: Additional dependencies [macos-15-intel]
+        if: ${{ matrix.os == 'macos-15-intel' }}
         run: python -m pipenv install macholib
 
-      - name: Build [macos-13]
-        if: ${{ matrix.os == 'macos-13' }}
+      - name: Build [macos-15-intel]
+        if: ${{ matrix.os == 'macos-15-intel' }}
         run: python -m pipenv run ./build_mac_installer.sh
 
-      - name: Upload a Build Artifact [macos-13]
+      - name: Upload a Build Artifact [macos-15-intel]
         uses: actions/upload-artifact@v4
-        if: ${{ matrix.os == 'macos-13' }}
+        if: ${{ matrix.os == 'macos-15-intel' }}
         with:
           name: shortcircuit.app.tar.gz
           path: ./dist/shortcircuit.app.tar.gz