chore: add risk disclaimer, CI pipeline, pytest coverage config

- Add risk warning after backtest demo output (survivorship bias, overfitting)
- Add disclaimer section at README end
- Add slippage comment to backtest demo command
- Add CI workflow (test + lint + mypy) for PR/push to main
- Add pytest config with coverage threshold (fail_under=50, current=56%)

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.12", "3.13"]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      - run: pip install -e ".[dev]"
+      - run: python -m pytest tests/unit/ -v --tb=short
+      - run: ruff check src/ tests/
+      - run: ruff format --check src/ tests/
+
+  mypy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+      - run: pip install -e ".[dev]"
+      - run: mypy src/