block pull requests to `stable` (#6001)

* block pull requests to `stable`

Pull requests should be opened against the `unstable` branch.
See https://nimbus.guide/contribute.html#build-and-deploy

* avoid deleting branch to mimic manual close

* avoid triggering on dependabot PRs

.github/workflows/ci.yml

@@ -15,6 +15,8 @@ on:
       - unstable
   pull_request:
     paths-ignore: ['media/**', 'docs/**', '**/*.md']
+    branches-ignore:
+      - stable
   workflow_dispatch:
 
 concurrency: # Cancel stale PR builds (but not push builds)

.github/workflows/pr_block.yml

@@ -0,0 +1,30 @@
+# beacon_chain
+# Copyright (c) 2024 Status Research & Development GmbH
+# Licensed and distributed under either of
+#   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
+#   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+name: PR block
+on:
+  pull_request:
+    branches:
+      - stable
+
+jobs:
+  close:
+    name: "Close pull request"
+    runs-on: ubuntu-latest
+    if: github.actor != 'dependabot[bot]'
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Close
+        run: |
+          gh pr close ${{ github.event.pull_request.number }} --comment \
+          'Pull requests to the `stable` branch are not allowed.
+          Please open your pull request against the `unstable` branch.
+          See https://nimbus.guide/contribute.html#build-and-deploy'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}