ci: add security workflows (CodeQL, Scorecard, Dependabot)

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    ignore:
+      - dependency-name: "*"
+        update-types:
+          - "version-update:semver-major"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5

.github/workflows/codeql.yml

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+name: CodeQL
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+  schedule:
+    - cron: '15 3 * * 0'
+
+permissions:
+  actions: read
+  contents: read
+  security-events: write
+
+env:
+  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: "true"
+
+jobs:
+  analyze:
+    name: Analyze (python)
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v7
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v4
+        with:
+          languages: python
+          queries: +security-extended
+
+      - name: Autobuild
+        uses: github/codeql-action/autobuild@v4
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v4
+        continue-on-error: true
+        with:
+          category: /language:python

.github/workflows/scorecard.yml

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+name: OpenSSF Scorecard
+
+on:
+  branch_protection_rule:
+  schedule:
+    - cron: '30 4 * * 1'
+  push:
+    branches:
+      - main
+
+permissions: read-all
+
+jobs:
+  scorecard:
+    runs-on: ubuntu-latest
+    permissions:
+      security-events: write
+      id-token: write
+      contents: read
+      actions: read
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+        with:
+          persist-credentials: false
+          fetch-depth: 0
+
+      - name: Run Scorecard
+        uses: ossf/scorecard-action@4eaacf0543bb3f2c246792bd56e8cdeffafb205a # v2.4.3
+        with:
+          results_file: scorecard-results.sarif
+          results_format: sarif
+          # publish_results requires a public repo; flip to true after the
+          # 2026-06-23 public launch.
+          publish_results: false
+
+      - name: Upload SARIF
+        uses: github/codeql-action/upload-sarif@v4
+        with:
+          sarif_file: scorecard-results.sarif