Add workflow to sync with upstream repository

.github/workflows/sync_upstream.yml

@@ -0,0 +1,21 @@
+name: Sync Upstream
+on:
+  schedule:
+    - cron: '0 6 * * 1'  # weekly, Monday 6am UTC
+  workflow_dispatch:
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          fetch-depth: 0
+
+      - name: Sync upstream
+        run: |
+          git remote add upstream https://github.com/gchq/CyberChef.git
+          git fetch upstream
+          git merge upstream/master
+          git push origin master