Add Sentinel CI workflow for workflow security scanning

Part of org-wide sentinel rollout. Warn-only mode (fail-on-findings: false).
Spec: https://www.notion.so/copilotkit/3683aa381852818bacd8e14eb7233c22

Author: jpr5

.github/workflows/sentinel.yml

@@ -0,0 +1,19 @@
+name: Sentinel
+on:
+  pull_request:
+  push:
+    branches: [main]
+permissions:
+  contents: read
+jobs:
+  scan:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - uses: jpr5/sentinel@v1
+        with:
+          severity: high
+          fail-on-findings: false