security: add Gitleaks secret scanning

.github/workflows/gitleaks.yml

@@ -0,0 +1,27 @@
+name: gitleaks
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pull-requests: read
+
+jobs:
+  scan:
+    name: scan
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Run Gitleaks
+        uses: gitleaks/gitleaks-action@v2
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.gitleaks.toml

@@ -0,0 +1,5 @@
+# Gitleaks config - extend the default ruleset.
+# Add repo-specific allowlist entries only after triaging confirmed false positives.
+
+[extend]
+useDefault = true