ci: add automated vulnerability scanning workflow

Add monthly scheduled vulnerability scans using govulncheck to detect
known security issues in Go dependencies and code.

- Run on push/PR to main branch
- Monthly scheduled scan (1st of each month at 2 AM)
- Manual trigger support via workflow_dispatch
- Uses golang/govulncheck-action for comprehensive scanning

Fixes #55

Author: sgaunet

.github/workflows/vulnerability-scan.yml

@@ -0,0 +1,34 @@
+name: Vulnerability Scan
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+  schedule:
+    - cron: '0 2 1 * *'  # Run at 2 AM on the 1st of every month
+  workflow_dispatch:     # Allow manual triggering
+
+permissions:
+  contents: read
+  security-events: write
+
+jobs:
+  vulnerability-scan:
+    runs-on: ubuntu-latest
+    name: Run govulncheck
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v5
+        with:
+          persist-credentials: false
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: 'go.mod'
+
+      - name: Run govulncheck
+        uses: golang/govulncheck-action@v1
+        with:
+          go-package: ./...