Add security scanning and Dependabot configuration

- Add security.yml workflow with Brakeman (SAST) and Bundle Audit
- Add dependabot.yml for automated dependency updates
- Schedule weekly security scans on Mondays
- Group dependency updates by type (development/production)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: koji-takao-sp

.github/dependabot.yml

@@ -0,0 +1,27 @@
+version: 2
+updates:
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 10
+    groups:
+      development-dependencies:
+        dependency-type: "development"
+      production-dependencies:
+        dependency-type: "production"
+    commit-message:
+      prefix: "deps"
+    labels:
+      - "dependencies"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    commit-message:
+      prefix: "ci"
+    labels:
+      - "ci"

.github/workflows/security.yml

@@ -0,0 +1,42 @@
+name: Security
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: ['**']
+  schedule:
+    - cron: '0 0 * * 1'  # Every Monday at 00:00 UTC
+
+jobs:
+  brakeman:
+    name: Brakeman (Static Analysis)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+      - name: Install Brakeman
+        run: gem install brakeman
+      - name: Run Brakeman
+        run: brakeman --no-pager -w2 --skip-files test/
+
+  bundle-audit:
+    name: Bundle Audit (Dependency Check)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+      - name: Install dependencies
+        run: bundle install --jobs 4 --retry 3
+      - name: Install bundle-audit
+        run: gem install bundler-audit
+      - name: Update vulnerability database
+        run: bundle-audit update
+      - name: Run bundle-audit
+        run: bundle-audit check