fix: replace cp --exclude with tar --exclude in backup workflow

Author: opencode

.github/workflows/backup.yml

@@ -26,10 +26,12 @@ jobs:
       - name: Create full tarball
         run: |
           DATE=$(date +%Y-%m-%d)
-          mkdir -p /tmp/mochios-backup
-          cp -r --exclude='.git' --exclude='bin' --exclude='guest' --exclude='mochi-engine' . /tmp/mochios-backup/mochios-$DATE/
-          tar czf "mochios-$DATE.tar.gz" -C /tmp/mochios-backup mochios-$DATE/
-          rm -rf /tmp/mochios-backup
+          tar czf "mochios-$DATE.tar.gz" \
+            --exclude='.git' \
+            --exclude='bin' \
+            --exclude='guest' \
+            --exclude='mochi-engine' \
+            .
 
       - name: Upload bundle artifact
         uses: actions/upload-artifact@v4