diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..ed53fd5
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,87 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  # ── Job 1: Lint ────────────────────────────────────────────────────
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4
+      
+      # TODO ②: Set up Python 3.11 using actions/setup-python
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: 3.11
+
+      - name: Cache pip
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/pip
+          key: ${{ runner.os }}-pip-${{ hashFiles('requirements.txt') }}
+          restore-keys: ${{ runner.os }}-pip-
+
+      - name: Install linter
+        run: pip install ruff
+
+      - name: Run lint
+        run: ruff check src/
+
+  # ── Job 2: Test ────────────────────────────────────────────────────
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.11", "3.12"]
+
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install dependencies
+        run: pip install -r requirements.txt
+
+      - name: Run tests
+        run: pytest src/ --junitxml=test-results.xml
+
+      # TODO ⑤: Upload test-results.xml as an artifact (always, even on failure)
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+            name: test-results-${{ matrix.python-version }}
+            path: test-results.xml
+
+  # ── Job 3: Build ───────────────────────────────────────────────────
+  build:
+    name: Build
+    needs: [lint, test]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4
+
+      - name: Build distribution
+        run: |
+          pip install build
+          cd src
+          python -m build --outdir ../dist
+
+      - name: Upload dist artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: dist
+          path: dist/
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