ci: enable squash auto-merge on ready PRs (PLAT-108) (#17)

* ci: enable squash auto-merge on ready PRs

* fix(ci): serialize auto-merge enable and ignore already-enabled

.github/workflows/auto-merge.yaml

@@ -0,0 +1,44 @@
+name: Enable auto-merge
+
+on:
+  pull_request:
+    types: [opened, ready_for_review, reopened, synchronize]
+    branches: [main]
+
+concurrency:
+  group: enable-auto-merge-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  enable:
+    if: >-
+      github.event.pull_request.draft == false
+      && github.event.pull_request.head.repo.full_name == github.repository
+      && github.event.pull_request.user.login != 'dependabot[bot]'
+      && github.event.pull_request.user.login != 'renovate[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Enable squash auto-merge
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR: ${{ github.event.pull_request.number }}
+          REPO: ${{ github.repository }}
+        run: |
+          set -euo pipefail
+          if [ "$(gh pr view "$PR" --repo "$REPO" --json autoMergeRequest --jq '.autoMergeRequest != null')" = "true" ]; then
+            echo "auto-merge already enabled"
+            exit 0
+          fi
+          if ! out=$(gh pr merge "$PR" --repo "$REPO" --auto 2>&1); then
+            echo "$out"
+            if [ "$(gh pr view "$PR" --repo "$REPO" --json autoMergeRequest --jq '.autoMergeRequest != null')" = "true" ]; then
+              echo "auto-merge already enabled"
+              exit 0
+            fi
+            exit 1
+          fi
+          printf '%s\n' "$out"