Add CI check that fails on merge conflict markers

.github/workflows/ci.yml

@@ -10,6 +10,17 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+
+      - name: Reject merge conflict markers
+        run: |
+          if git grep -nE '^(<<<<<<<|>>>>>>>|=======)' -- . \
+            ':(exclude)*.lock' \
+            ':(exclude)package-lock.json'; then
+            echo "::error::Merge conflict markers found in tracked files"
+            exit 1
+          fi
+          echo "No conflict markers found"
+
       - uses: actions/setup-node@v4
         with:
           node-version: "22"