ci: add git-secrets GitHub Actions workflow

Scans full commit history on every push and PR.
Local hooks are not committed - this enforces scanning in CI.

Author: loki-bedlam

.github/workflows/secret-scan.yml

@@ -0,0 +1,26 @@
+name: Secret Scan
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+    branches: ["**"]
+
+jobs:
+  git-secrets:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install git-secrets
+        run: |
+          git clone https://github.com/awslabs/git-secrets.git /tmp/git-secrets
+          cd /tmp/git-secrets && sudo make install
+
+      - name: Register AWS patterns
+        run: git secrets --register-aws --global
+
+      - name: Scan for secrets
+        run: git secrets --scan-history