fix: correct --head branch format and use --enable-auto-merge (GRO-178)

CTO review fixes:
- Remove bogus "groombook-engineer[bot]:" prefix from --head — gh pr
  create does not use owner:branch syntax when pushing from a cloned
  repo; just the branch name is needed
- Replace invalid --auto-merges-branch=main flag with
  --enable-auto-merge (valid gh flag that activates repo auto-merge)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -342,8 +342,8 @@ jobs:
           gh pr create \
             --repo groombook/infra \
             --base main \
-            --head "groombook-engineer[bot]:chore/update-image-tags-${TAG}" \
+            --head "chore/update-image-tags-${TAG}" \
             --title "chore: update image tags to ${TAG}" \
             --body "[GRO-178](/d50d9792/issues/GRO-178) — automated image tag update from main merge" \
-            --auto-merges-branch=main \
+            --enable-auto-merge \
             2>&1 || echo "PR creation attempted"