Add codeql workflow

.github/workflows/codeql.yml

@@ -0,0 +1,46 @@
+name: "CodeQL Advanced"
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+  schedule:
+    - cron: '26 15 * * 3'
+
+jobs:
+  analyze:
+    name: Analyze (${{ matrix.language }})
+    runs-on: ${{ (matrix.language == 'swift' && 'macos-latest') || 'ubuntu-latest' }}
+    permissions:
+      security-events: write
+      packages: read
+      actions: read
+      contents: read
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+        - language: actions
+          build-mode: none
+        - language: swift
+          build-mode: manual
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v3
+      with:
+        languages: ${{ matrix.language }}
+        build-mode: ${{ matrix.build-mode }}
+    - if: matrix.build-mode == 'manual'
+      name: "Select Xcode"
+      run: sudo xcrun xcode-select -s /Applications/Xcode_26.0.app
+    - if: matrix.build-mode == 'manual'
+      name: "Build"
+      run: xcrun xcodebuild -project Sources/Secretive.xcodeproj -scheme Secretive build
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v3
+      with:
+        category: "/language:${{matrix.language}}"