ci: add CodeQL workflow, switch release to release-please, simplify CI

- ci.yml: lint + bandit (PyCQA/bandit-action) + pytest test/backtesting/
- release.yml: switch from manual tag-based to release-please auto-changelog
- codeql.yml: new weekly + on-push CodeQL Python analysis

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/codeql.yml

@@ -0,0 +1,31 @@
+name: CodeQL
+
+on:
+  push:
+    branches: [master, main]
+  pull_request:
+    branches: [master, main]
+  schedule:
+    - cron: "0 6 * * 1"  # Every Monday at 06:00 UTC
+
+permissions:
+  contents: read
+  security-events: write
+  actions: read
+
+jobs:
+  analyze:
+    name: Analyze (Python)
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: github/codeql-action/init@v3
+        with:
+          languages: python
+          queries: security-and-quality
+
+      - uses: github/codeql-action/analyze@v3
+        with:
+          category: "/language:python"