ci(benchmarks): open a PR with the refreshed README instead of pushing to main

.github/workflows/benchmark.yml

@@ -11,14 +11,15 @@ on:
         description: 'Version label to record (defaults to package.json version)'
         required: false
         type: string
-      commit:
-        description: 'Commit the refreshed README back to the default branch'
+      open_pr:
+        description: 'Open a PR with the refreshed README'
         required: false
         default: true
         type: boolean
 
 permissions:
   contents: write
+  pull-requests: write
 
 jobs:
   benchmark:
@@ -61,7 +62,7 @@ jobs:
           LAMBDA_API_VERSION: ${{ steps.ver.outputs.value }}
         run: node run.js --md results/RESULTS.md --json results/raw.json --update-readme
 
-      # Keep the committed README Prettier-clean so the push doesn't break main's CI.
+      # Keep the committed README Prettier-clean so the PR doesn't fail main's CI.
       - name: Format README
         run: npx --yes prettier@2 --write README.md
 
@@ -74,15 +75,48 @@ jobs:
       - name: Add results to job summary
         run: cat benchmarks/results/RESULTS.md >> "$GITHUB_STEP_SUMMARY"
 
-      - name: Commit refreshed README
-        if: ${{ github.event_name == 'release' || inputs.commit }}
+      # The default branch doesn't accept direct pushes, so the refreshed README goes
+      # through a PR. Each run gets its own branch so concurrent or repeated runs never
+      # clobber one another.
+      - name: Open PR with refreshed README
+        if: ${{ github.event_name == 'release' || inputs.open_pr }}
+        env:
+          GH_TOKEN: ${{ github.token }}
+          BASE: ${{ github.event.repository.default_branch }}
+          VERSION: ${{ steps.ver.outputs.value }}
         run: |
           if git diff --quiet -- README.md; then
-            echo 'README.md unchanged — nothing to commit'
+            echo 'README.md unchanged — no PR needed'
             exit 0
           fi
+
+          LABEL="${VERSION:-manual}"
+          BRANCH="benchmarks/refresh-${LABEL}-${GITHUB_RUN_ID}"
+
           git config user.name 'github-actions[bot]'
           git config user.email 'github-actions[bot]@users.noreply.github.com'
+          git switch -c "$BRANCH"
           git add README.md
-          git commit -m 'docs: update benchmark results [skip ci]'
-          git push origin HEAD:${{ github.event.repository.default_branch }}
+          git commit -m "docs: update benchmark results for ${LABEL}"
+          git push origin "$BRANCH"
+
+          {
+            echo "Automated refresh of the Benchmarks section of the README, generated by [this run](${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/actions/runs/${GITHUB_RUN_ID})."
+            echo
+            echo "Version label: \`${LABEL}\` · Trigger: \`${GITHUB_EVENT_NAME}\`"
+            echo
+            echo "Perf numbers come from a shared GitHub-hosted runner and are noisy between runs, so treat"
+            echo "movement within a few percent as noise rather than a regression."
+            echo
+            echo '<details><summary>Full results</summary>'
+            echo
+            cat benchmarks/results/RESULTS.md
+            echo
+            echo '</details>'
+          } > /tmp/pr-body.md
+
+          gh pr create \
+            --base "$BASE" \
+            --head "$BRANCH" \
+            --title "docs: refresh benchmark results for ${LABEL}" \
+            --body-file /tmp/pr-body.md