ci: add GitHub Actions workflow running tests on Python 3.10/3.11/3.12

Author: byfor8

.github/workflows/tests.yml

@@ -0,0 +1,34 @@
+name: tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.11", "3.12"]
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+      - name: Run unit tests
+        run: python -m unittest tests.test_backtest -v
+      - name: Smoke test backtest engine
+        run: python backtest.py > /dev/null
+      - name: Smoke test regime comparison
+        run: python compare_regime_scenarios.py > /dev/null
+      - name: Smoke test live_demo (mock fallback)
+        run: python live_demo.py --json > /dev/null
+      - name: Validate skill.yaml syntax
+        run: python -c "import yaml; yaml.safe_load(open('skill.yaml'))"