feat: Add action to lock Threads (#21)

Author: mkilchhofer

.github/workflows/lock.yaml

@@ -0,0 +1,28 @@
+name: lock-threads
+on:
+  # run daily
+  schedule:
+    - cron: '0 0 * * *'
+
+  # allow manual trigger
+  workflow_dispatch:
+
+permissions:
+  issues: write
+  pull-requests: write
+
+# Ensure only one lock action can run at a time
+concurrency:
+  group: lock
+
+jobs:
+  lock:
+    if: github.repository == 'swisspost/terraform-grafana-prometheus-alerts'
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: dessant/lock-threads@1bf7ec25051fe7c00bdd17e6a7cf3d7bfb7dc771 # v5.0.1
+        with:
+          github-token: ${{ github.token }}
+          issue-inactive-days: 30
+          pr-inactive-days: 30
+          process-only: 'issues, prs'