ci: CLA signing check on pull requests

CLA Assistant Lite workflow: a PR stays red until its author signs
CLA.md by comment; signatures land on the cla-signatures branch.

.github/workflows/cla.yml

@@ -0,0 +1,29 @@
+# CLA Assistant Lite: blocks a PR until its author has signed CLA.md by
+# commenting the sign phrase; signatures land on the cla-signatures branch.
+name: CLA
+
+on:
+  issue_comment:
+    types: [created]
+  pull_request_target:
+    types: [opened, closed, synchronize]
+
+permissions:
+  actions: write
+  contents: write
+  pull-requests: write
+  statuses: write
+
+jobs:
+  cla:
+    runs-on: ubuntu-latest
+    if: (github.event.comment.body == 'recheck' || github.event.comment.body == 'I have read the CLA Document and I hereby sign the CLA') || github.event_name == 'pull_request_target'
+    steps:
+      - uses: contributor-assistant/github-action@v2.6.1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          path-to-signatures: 'signatures/cla.json'
+          path-to-document: 'https://github.com/stffnb/Web_ODF_Editor/blob/main/CLA.md'
+          branch: 'cla-signatures'
+          allowlist: 'stffnb,dependabot[bot],*[bot]'