ci: fail the linter while the README notice is present

The labeler converts a PR to a draft while README.md has the review
notice. Nothing else stops a merge. If the labeler fails, or somebody
merges before it runs, the PR goes in with the notice.

Let's add a linter job that fails when README.md has the notice. We keep
it out of the main lint job. That way it doesn't wait for the tools tree
build, and it can be made a required status check by itself.

(cherry picked from commit f1ea185bcf5f3f286c92b015294190b23f7215f5)

.github/workflows/linter.yml

@@ -14,6 +14,23 @@ permissions:
   contents: read
 
 jobs:
+  readme-notice:
+    name: Check for README notice
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Repo checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1
+        with:
+          persist-credentials: false
+
+      - name: Check that README.md has no review notice
+        run: |
+          if grep -F "Remove this line to confirm you've reviewed this PR before submitting." README.md >/dev/null; then
+              echo "::error file=README.md,line=1::Remove the review notice from README.md before merging."
+              exit 1
+          fi
+
   build:
     name: Lint Code Base
     runs-on: ubuntu-24.04