ci: add workflow to label merge conflicts (#1269)

Add a workflow to label merge conflicts, making it more obvious when PRs
have a merge conflicts. This is also used by Home Manager and Nixpkgs.

Link: https://github.com/nix-community/stylix/pull/1269

Co-authored-by: pancho horrillo <pancho@pancho.name>
Reviewed-by: eveeifyeve <88671402+Eveeifyeve@users.noreply.github.com>
Reviewed-by: Matt Sturgeon <matt@sturgeon.me.uk>
Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com>

.github/workflows/conflicts.yml

@@ -0,0 +1,27 @@
+---
+name: Label Merge Conflicts
+
+on:  # yamllint disable-line rule:truthy
+  push:
+    branches:
+      - master
+      - release-**
+
+jobs:
+  conflicts:
+    runs-on: ubuntu-24.04
+    if: github.repository_owner == 'nix-community'
+    steps:
+      - uses: actions/create-github-app-token@v2
+        id: app-token
+        with:
+          app-id: ${{ vars.APP_ID }}
+          private-key: ${{ secrets.APP_PRIVATE_KEY }}
+          permission-contents: read
+          permission-pull-requests: write
+      - uses: mschilde/auto-label-merge-conflicts@v2
+        with:
+          GITHUB_TOKEN: ${{ steps.app-token.outputs.token }}
+          CONFLICT_LABEL_NAME: "status: merge conflict"
+          MAX_RETRIES: 5
+          WAIT_MS: 10000