fix(mac): default to universal binary for distribution builds

Closes #1393

The distribution script (package-mac-dist.sh) now defaults BUILD_ARCHS to 'all',
producing universal binaries that run natively on both Apple Silicon and Intel Macs.

Previously, the script inherited the host architecture default from package-mac-app.sh,
which meant release builds done on ARM Macs only included ARM binaries.

scripts/package-mac-dist.sh

@@ -10,6 +10,9 @@ set -euo pipefail
 
 ROOT_DIR="$(cd "$(dirname "$0")/.." && pwd)"
 
+# Default to universal binary for distribution builds (supports both Apple Silicon and Intel Macs)
+export BUILD_ARCHS="${BUILD_ARCHS:-all}"
+
 "$ROOT_DIR/scripts/package-mac-app.sh"
 
 APP="$ROOT_DIR/dist/Clawdbot.app"