Project scaffold and GitHub Actions CI

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+# A push that obsoletes a previous run cancels it.
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  lint:
+    name: ruff
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+          cache: pip
+          cache-dependency-path: pyproject.toml
+      - run: pip install --upgrade pip
+      - run: pip install ruff
+      - run: ruff check .
+
+  test:
+    name: pytest (py${{ matrix.python }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python: ["3.11", "3.12", "3.13"]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python }}
+          cache: pip
+          cache-dependency-path: pyproject.toml
+      - run: pip install --upgrade pip
+      # ``-e .[dev,plotting,excel]`` so every optional extra is exercised.
+      # Gurobi is not installable on free runners; the relevant tests
+      # skip themselves when ``optlang.gurobi_interface`` cannot import.
+      - run: pip install -e ".[dev,plotting,excel]"
+      - run: pytest -q --maxfail=5 --durations=20