ci: enable squash auto-merge on ready PRs (PLAT-108) (#234)

* ci: enable squash auto-merge on ready PRs

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

* fix(ci): enqueue merge queue as seahaven-bot

* fix(ci): re-arm auto-merge as seahaven-bot after failed enqueue

.github/workflows/auto-merge.yaml

@@ -0,0 +1,67 @@
+name: Enable auto-merge
+
+on:
+  pull_request:
+    types: [opened, ready_for_review, reopened, synchronize]
+    branches: [main]
+  pull_request_review:
+    types: [submitted]
+
+concurrency:
+  group: enable-auto-merge-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+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]'
+      && (github.event_name != 'pull_request_review' || github.event.review.state == 'approved')
+    runs-on: ubuntu-latest
+    steps:
+      - name: Mint seahaven-bot token
+        id: app-token
+        uses: actions/create-github-app-token@bcd2ba49218906704ab6c1aa796996da409d3eb1 # v3.2.0
+        with:
+          app-id: ${{ secrets.SEAHAVEN_BOT_APP_ID }}
+          private-key: ${{ secrets.SEAHAVEN_BOT_PRIVATE_KEY }}
+          owner: Sea-Haven-Industries
+          repositories: ${{ github.event.repository.name }}
+
+      - name: Enqueue or arm auto-merge
+        env:
+          GH_TOKEN: ${{ steps.app-token.outputs.token }}
+          PR: ${{ github.event.pull_request.number }}
+          REPO: ${{ github.repository }}
+          PR_NODE_ID: ${{ github.event.pull_request.node_id }}
+        run: |
+          set -euo pipefail
+          in_queue="$(gh api graphql -f query='query($id:ID!) { node(id:$id) { ... on PullRequest { isInMergeQueue } } }' -f id="$PR_NODE_ID" --jq '.data.node.isInMergeQueue')"
+          if [ "$in_queue" = "true" ]; then
+            echo "already in merge queue"
+            exit 0
+          fi
+          set +e
+          out="$(gh api graphql -f query='mutation($id:ID!) { enqueuePullRequest(input:{pullRequestId:$id}) { mergeQueueEntry { state } } }' -f id="$PR_NODE_ID" 2>&1)"
+          status=$?
+          set -e
+          if [ "$status" -eq 0 ]; then
+            printf '%s\n' "$out"
+            exit 0
+          fi
+          echo "$out"
+          # Re-arm as seahaven-bot. A leftover GITHUB_TOKEN auto-merge would
+          # enqueue without starting merge_group.
+          gh pr merge "$PR" --repo "$REPO" --disable-auto || true
+          if ! gh pr merge "$PR" --repo "$REPO" --auto; then
+            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