ci: add CodeQL workflow (language: actions)

.github/workflows/codeql.yml

@@ -0,0 +1,50 @@
+# SPDX-License-Identifier: MPL-2.0
+name: CodeQL Security Analysis
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+  schedule:
+    - cron: '0 6 1 * *'   # monthly 1st 06:00 UTC
+
+# Estate guardrail: cancel superseded runs so re-pushes / rebased PR
+# updates do not pile up queued runs against the shared account-wide
+# Actions concurrency pool. Applied only to read-only check workflows
+# (no publish/mutation), so cancelling a superseded run is always safe.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  analyze:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    permissions:
+      contents: read
+      security-events: write
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - language: actions
+            build-mode: none
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@c6f931105cb2c34c8f901cc885ba1e2e259cf745 # v3
+        with:
+          languages: ${{ matrix.language }}
+          build-mode: ${{ matrix.build-mode }}
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@c6f931105cb2c34c8f901cc885ba1e2e259cf745 # v3
+        with:
+          category: "/language:${{ matrix.language }}"