Stop writing the PR number into the size label artifact

.github/workflows/pr-size-labeler.yml

@@ -65,21 +65,14 @@ jobs:
       # This workflow runs on `pull_request`, so a pull request — including one
       # from a fork — supplies the definition that produces these files.
       # Nothing in them can be trusted to say which pull request to act on, so
-      # the consumer derives the number from its own trigger event instead.
-      #
-      # pr-number.txt is still written even though the new consumer ignores it.
-      # `workflow_run` consumers always execute the copy on the default branch,
-      # so until this change merges the consumer reading it is the old one, and
-      # dropping the file here would break labelling for every open pull
-      # request. Removing the write is a follow-up once this has landed.
+      # the consumer derives the number from its own trigger event instead, so
+      # the label is the only thing that needs to cross the boundary.
       - name: Save size label to artifact
         env:
           SIZE_LABEL: ${{ steps.size.outputs.result }}
-          PR_NUMBER: ${{ github.event.pull_request.number }}
         run: |
           mkdir -p pr-size
           echo "$SIZE_LABEL" > pr-size/label.txt
-          echo "$PR_NUMBER" > pr-size/pr-number.txt
 
       - name: Upload artifact
         uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6