ci: add Gitleaks secret scan

.github/workflows/secret-scan.yml

@@ -0,0 +1,27 @@
+###############################################################################
+# Secret scan
+#
+# Calls the public org scanner on main. Allow-list updates there apply on the
+# next run, with no pin bump in this repo. gitleaks git scans the full history.
+###############################################################################
+
+name: Secret scan
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+  push:
+    branches: [main]
+  schedule:
+    - cron: "0 3 1 * *"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  gitleaks:
+    name: Secret Scan (Gitleaks)
+    permissions:
+      contents: read
+    uses: m0-platform/secret-scan/.github/workflows/secret-scan.yml@main

.gitleaksignore

@@ -0,0 +1,3 @@
+# Reviewed false alarms. Each line is file:rule:start-line.
+# A new secret on a different line still fails the scan.
+test/utils/ERC1271WalletMock.sol:evm-32-byte-hex:18