chore: add arch to macOS DMG filename

Use artifactName template to always include architecture in DMG
filename (e.g. CodePilot-0.2.3-x64.dmg) for clarity.

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

electron-builder.yml

@@ -41,6 +41,7 @@ asarUnpack:
 mac:
   icon: build/icon.icns
   category: public.app-category.developer-tools
+  artifactName: "${productName}-${version}-${arch}.${ext}"
   target:
     - target: dmg
       arch: [arm64, x64]