CI: Automatically label PRs with merge conflicts

This action adds and removes the new "conflicts" label to indicate
whether a pull request is in need of conflict resolution. This both
automatically informs the author of the PR that this is the case, if
they have notifications enabled that is, and makes for an easier
evaluation of the PR queue.

.github/workflows/merge-conflict-labeler.yml

@@ -0,0 +1,22 @@
+name: 'Label PRs with merge conflicts'
+on:
+  # PRs typically get conflicted after a push to master.
+  push:
+    branches: [master]
+
+  # If a PR targeting master is (re)opened or updated, recheck for conflicts and update the label.
+  # NOTE: This runs against the target branch, not the PR branch.
+  pull_request_target:
+    types: [opened, synchronize, reopened]
+    branches: [master]
+
+jobs:
+  auto-labeler:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: mschilde/auto-label-merge-conflicts@591722e97f3c4142df3eca156ed0dcf2bcd362bd
+        with:
+          CONFLICT_LABEL_NAME: 'conflicts'
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          MAX_RETRIES: 3
+          WAIT_MS: 15000