Add GitHub actions

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - '*'
+  workflow_dispatch:
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  library:
+    name: macOS
+    strategy:
+      matrix:
+        xcode: ['16.3']
+        config: ['debug', 'release']
+    runs-on: macos-15
+    steps:
+      - uses: actions/checkout@v4
+      - name: Select Xcode ${{ matrix.xcode }}
+        run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode }}.app
+      - name: Run ${{ matrix.config }} tests
+        run: swift test -c ${{ matrix.config }}
+
+  linux:
+    name: Linux
+    strategy:
+      matrix:
+        swift:
+          - '6.1'
+    runs-on: ubuntu-latest
+    container: swift:${{ matrix.swift }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build
+        run: swift build

.github/workflows/format.yml

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+NB: Compatible swift-format requires Xcode 16.3, not yet available on GitHub
+name: Format
+
+on:
+  push:
+    branches:
+      - main
+
+concurrency:
+  group: format-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  swift_format:
+    name: swift-format
+    runs-on: macos-15
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+      - name: Select Xcode 16.3
+        run: sudo xcode-select -s /Applications/Xcode_16.3.app
+      - name: Format
+        run: make format
+      - uses: stefanzweifel/git-auto-commit-action@v5
+        with:
+          commit_message: Run swift-format
+          branch: 'main'