ci: block Co-authored-by Claude/Anthropic trailers on PRs

New GitHub Actions check that scans every PR commit for
`Co-authored-by: ... claude ...` or `... anthropic ...` trailers and
fails the PR with a clear remediation message if found. Contributors
can still use AI tools; the trailer attribution must be removed before
the PR is eligible to merge, consistent with the project contributor
guidelines.

The workflow scans only commits introduced by the PR
(base.sha..head.sha), so existing history is untouched.

.github/workflows/block-claude-coauthor.yml

@@ -0,0 +1,43 @@
+name: Block Claude / Anthropic co-author trailers
+
+# Rejects PRs that contain a `Co-authored-by: ... claude ...` or `... anthropic ...`
+# trailer in any of their commits. Contributors can still use AI tools to help
+# write code, but they must remove the co-author attribution before the PR is
+# eligible to merge, per the project's contributor guidelines.
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Scan PR commits for disallowed co-author trailers
+        run: |
+          BASE_SHA="${{ github.event.pull_request.base.sha }}"
+          HEAD_SHA="${{ github.event.pull_request.head.sha }}"
+          FOUND=0
+          while IFS= read -r c; do
+            [ -z "$c" ] && continue
+            SUBJECT=$(git log -1 "$c" --format=%s)
+            if git log -1 "$c" --format="%B" | grep -qiE 'co-authored-by:.*(claude|anthropic)'; then
+              echo "::error::Commit $c ($SUBJECT) has a Claude/Anthropic co-author trailer. Please remove it before this PR can merge."
+              FOUND=1
+            fi
+          done < <(git log --format=%H "$BASE_SHA".."$HEAD_SHA")
+          if [ "$FOUND" != "0" ]; then
+            echo ""
+            echo "How to fix:"
+            echo "  1. Run: git rebase -i $BASE_SHA"
+            echo "  2. Mark each flagged commit as 'reword'"
+            echo "  3. Delete the Co-authored-by line from the commit message"
+            echo "  4. Save, then: git push --force-with-lease"
+            exit 1
+          fi
+          echo "No disallowed co-author trailers found."