ci: add GitHub Actions test workflow (Python 3.11–3.13)

- Add .github/workflows/ci.yml with correct placement in muxplex repo
- Matrix: Python 3.11, 3.12, 3.13 with fail-fast: false
- Uses uv for Python and dependency management with package caching
- Runs pytest via uv run pytest -v (config in pyproject.toml addopts)
- timeout-minutes: 15 to prevent runaway CI costs
- permissions: contents: read for minimal GITHUB_TOKEN scope

Co-authored-by: Amplifier <amplifier@example.com>

.github/workflows/ci.yml

@@ -0,0 +1,35 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.11", "3.12", "3.13"]
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v4
+        with:
+          enable-cache: true
+
+      - name: Set up Python ${{ matrix.python-version }}
+        run: uv python install ${{ matrix.python-version }}
+
+      - name: Install dependencies
+        run: uv sync --extra dev
+
+      - name: Run tests
+        run: uv run pytest -v