security: add gitleaks CI secret scanning

Author: dialupdisaster

.github/workflows/secret-scan.yml

@@ -0,0 +1,23 @@
+name: Secret Scan
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+jobs:
+  gitleaks:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Run gitleaks
+        uses: gitleaks/gitleaks-action@v2
+        env:
+          GITLEAKS_ENABLE_UPLOAD_ARTIFACT: false

CONTRIBUTING.md

@@ -70,6 +70,15 @@ cargo test
 
 - If behavior changes, update docs in the same PR (`README.md`, `AGENTS.md`, this file, or `SECURITY.md` when relevant).
 
+## Secret Scanning
+
+- CI runs `gitleaks` on pushes and pull requests.
+- Before opening a PR, run a local scan if `gitleaks` is installed:
+
+```bash
+gitleaks git --no-banner
+```
+
 ## Commit Guidelines
 
 Use Conventional Commits where possible: