ci: add GitHub Actions CI (build + test + lint) (#31)

- Matrix: OCaml 5.2/5.3 on ubuntu+macos, 5.4 on ubuntu
- Build + test with dune
- Lint: verify opam files stay in sync with dune-project

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jeong-sik

.github/workflows/ci.yml

@@ -0,0 +1,57 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: Build and Test
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+        ocaml-compiler: ['5.2', '5.3']
+        include:
+          - os: ubuntu-latest
+            ocaml-compiler: '5.4'
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up OCaml ${{ matrix.ocaml-compiler }}
+        uses: ocaml/setup-ocaml@v3
+        with:
+          ocaml-compiler: ${{ matrix.ocaml-compiler }}
+
+      - name: Install dependencies
+        run: opam install . --deps-only --with-test
+
+      - name: Build
+        run: opam exec -- dune build
+
+      - name: Run tests
+        run: opam exec -- dune runtest
+
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up OCaml
+        uses: ocaml/setup-ocaml@v3
+        with:
+          ocaml-compiler: '5.4'
+
+      - name: Install dependencies
+        run: opam install . --deps-only
+
+      - name: Check opam files are up to date
+        run: opam exec -- dune build @install && git diff --exit-code *.opam