Add local testing instructions and GitHub Actions CI to run pytest

.github/workflows/ci.yml

@@ -0,0 +1,26 @@
+name: CI
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e . pytest
+
+      - name: Run tests
+        run: python -m pytest -q

README.md

@@ -218,6 +218,23 @@ Features will be added **only if they do not weaken these constraints**.
 
 ---
 
+## Testing
+
+Install the package in editable mode so tests can import `determinant` directly:
+
+```bash
+python -m pip install -e .
+python -m pytest -q
+```
+
+If you prefer not to install in editable mode, set `PYTHONPATH` to the repo root:
+
+```bash
+PYTHONPATH=. python -m pytest -q
+```
+
+---
+
 ## Non-Negotiables
 
 * No hidden state