fix(ci): add CodeQL workflow for fork pull requests

GitHub's default CodeQL setup only runs on PRs from the same repository.
Fork PRs (like #72) are skipped because pull_request events from forks
don't have write access to the base repo's security events.

Add an explicit codeql.yml workflow using pull_request_target so CodeQL
analysis runs for both fork and non-fork PRs. The workflow checks out
the PR head SHA explicitly while keeping permissions minimal
(security-events: write, contents: read only).

Author: bug-ops

.github/workflows/codeql.yml

@@ -0,0 +1,42 @@
+name: CodeQL
+
+on:
+  push:
+    branches: [main]
+  pull_request_target:
+    branches: [main]
+  schedule:
+    - cron: "0 6 * * 1"
+
+permissions: {}
+
+jobs:
+  analyze:
+    name: Analyze (${{ matrix.language }})
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    permissions:
+      security-events: write
+      contents: read
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [actions, rust]
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v3
+        with:
+          languages: ${{ matrix.language }}
+          queries: security-extended
+
+      - name: Autobuild
+        uses: github/codeql-action/autobuild@v3
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v3
+        with:
+          category: "/language:${{ matrix.language }}"