Add GitHub Actions CI workflow

Summary:
Adds a GitHub Actions CI workflow for the tintype OSS repo
(github.com/facebookincubator/tintype). The workflow builds and tests the
library on the Ubuntu + macOS × Python 3.12/3.13/3.14 matrix, installing the
required libzstd system dep and exercising the pybind11 C++17 extension build
via `pip install -v .`.

Key choices:
- `fail-fast: false` so every matrix combination reports independently.
- `allow-prereleases: true` on setup-python to support Python 3.14.
- `cache: 'pip'` on setup-python to reduce per-job install time.
- Workflow-level `permissions: contents: read` for least-privilege GITHUB_TOKEN.
- `timeout-minutes: 30` to fail fast on hung runs.
- Concurrency group cancels superseded PR runs.

The workflow file lands in the OSS repo via ShipIt once the companion
configerator change exports the .github/ directory.

Reviewed By: aperez

Differential Revision: D102836163

fbshipit-source-id: 0aed6e8f5d0ccec40c35bc2c939b841a4ad83417

Author: pdepetro

.github/workflows/ci.yml

@@ -0,0 +1,56 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build-and-test:
+    name: build-and-test (${{ matrix.os }}, py${{ matrix.python-version }})
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+        python-version: ["3.12", "3.13", "3.14"]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          allow-prereleases: true
+          cache: 'pip'
+
+      - name: Install zstd (Linux)
+        if: runner.os == 'Linux'
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libzstd-dev
+
+      - name: Install zstd (macOS)
+        if: runner.os == 'macOS'
+        run: brew install zstd
+
+      - name: Upgrade pip
+        run: python -m pip install --upgrade pip
+
+      - name: Install tintype
+        run: pip install -v .
+
+      - name: Install test dependencies
+        run: pip install pytest
+
+      - name: Run tests
+        run: pytest tests/ -v