Add GitHub Actions CI workflow

Runs pytest on every push and pull request using Python 3.11.

https://claude.ai/code/session_01VQfqug9MDEyRFydkmES4n2

Author: claude

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+          cache: pip
+
+      - run: pip install -r requirements.txt
+
+      - run: pytest tests/ -v