ci: enable squash auto-merge on ready PRs (PLAT-108) (#115)

* ci: enable squash auto-merge on ready PRs

* fix(ci): grant contents read so auto-merge can query the PR

* fix(ci): restore contents write for enablePullRequestAutoMerge

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

* fix(ci): request squash auto-merge so PRs enter the merge queue

.github/workflows/auto-merge.yaml

@@ -0,0 +1,46 @@
+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: ${{ github.repository }}
+        run: |
+          set -euo pipefail
+          method="$(gh pr view "$PR" --repo "$REPO" --json autoMergeRequest --jq '.autoMergeRequest.mergeMethod // empty')"
+          # Merge queue requires squash. `gh pr merge --auto` with no method
+          # records MERGE and never enqueues.
+          if [ "$method" = "MERGE" ] || [ "$method" = "REBASE" ]; then
+            gh pr merge "$PR" --repo "$REPO" --disable-auto
+          fi
+          if ! out=$(gh pr merge "$PR" --repo "$REPO" --squash --auto 2>&1); then
+            echo "$out"
+            if [ "$(gh pr view "$PR" --repo "$REPO" --json autoMergeRequest --jq '.autoMergeRequest.mergeMethod // empty')" = "SQUASH" ]; then
+              echo "squash auto-merge already enabled"
+              exit 0
+            fi
+            exit 1
+          fi
+          printf '%s\n' "$out"