Security: add detect-secrets scan

.github/workflows/ci.yml

@@ -141,6 +141,31 @@ jobs:
       - name: Run ${{ matrix.task }} (${{ matrix.runtime }})
         run: ${{ matrix.command }}
 
+  secrets:
+    runs-on: blacksmith-4vcpu-ubuntu-2404
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          submodules: false
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install detect-secrets
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install detect-secrets==1.5.0
+
+      - name: Detect secrets
+        run: |
+          if ! detect-secrets scan --baseline .secrets.baseline; then
+            echo "::error::Secret scanning failed. See docs/gateway/security.md#secret-scanning-detect-secrets"
+            exit 1
+          fi
+
   checks-windows:
     runs-on: blacksmith-4vcpu-windows-2025
     defaults: