[codex] Add security automation baseline

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+      day: monday
+      time: "09:00"
+      timezone: America/Phoenix
+    labels:
+      - dependencies
+      - github-actions

.github/workflows/code-scanning.yml

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+name: Code Scanning
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  schedule:
+    - cron: "34 9 * * 1"
+
+permissions:
+  contents: read
+  security-events: write
+
+jobs:
+  semgrep:
+    name: Semgrep PHP
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install Semgrep
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install semgrep
+
+      - name: Run Semgrep
+        run: semgrep scan --config p/php --sarif --output semgrep.sarif
+
+      - name: Upload SARIF
+        uses: github/codeql-action/upload-sarif@v3
+        with:
+          sarif_file: semgrep.sarif

SECURITY.md

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+# Security Policy
+
+## Reporting A Vulnerability
+
+Please report vulnerabilities through GitHub private vulnerability reporting:
+
+https://github.com/acodebeard/apod/security/advisories/new
+
+Do not open a public issue for suspected vulnerabilities.
+
+## Supported Version
+
+This repository tracks the public APOD recreation source on the `main` branch. Security fixes are handled against `main`.