chore: add pr lockdown

.github/workflows/lockdown.yml

@@ -0,0 +1,23 @@
+name: 'Repo Lockdown'
+
+on:
+  pull_request_target:
+    types: opened
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  action:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/repo-lockdown@v3
+        with:
+          pr-comment: 'Hi! Thank you for your contribution.
+
+          This repo is only a mirror and unfortunately we can''t accept PRs made here. Please re-submit your changes to [our Gitea instance](https://kolaente.dev/vikunja/api/pulls).
+
+          Also check out the [contribution guidelines](https://vikunja.io/docs/development/#pull-requests).
+
+          Thank you for your understanding.'