ci: add check to block AI agent Co-authored-by trailers in PRs

Add a GitHub Actions workflow and shell script that scan all commits
in a pull request for Co-authored-by trailers from known AI coding
agents (Claude, Cursor, Copilot, Codex, Aider, Devin, Gemini, Jules,
Windsurf, Cline, Amazon Q, Continue, OpenCode, etc.).

The check fails with clear instructions on how to remove the trailers
via interactive rebase.

.github/workflows/check-ai-co-authors.yml

@@ -0,0 +1,19 @@
+name: Check AI Co-Authors
+
+on:
+  pull_request:
+    branches: ['*']
+
+jobs:
+  check-ai-co-authors:
+    name: Check for AI agent co-author trailers
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Check commits for AI co-author trailers
+        run: bash .github/scripts/check-ai-co-authors.sh "${{ github.event.pull_request.base.sha }}" "${{ github.event.pull_request.head.sha }}"