Add CI workflow for automated testing

- Run tests on Python 3.10, 3.11, 3.12
- Check code formatting with Black and isort
- Use uv for fast dependency installation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: MaxGhenis

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+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 uv
+        uses: astral-sh/setup-uv@v5
+
+      - name: Install dependencies
+        run: |
+          uv pip install --system -e ".[dev]"
+
+      - name: Run tests
+        run: |
+          uv run pytest tests/ -v --tb=short
+
+      - name: Check formatting
+        if: matrix.python-version == '3.10'
+        run: |
+          pip install black isort
+          black --check givecalc/ tests/ ui/ --line-length 79
+          isort --check givecalc/ tests/ ui/