Add GitHub Actions CI workflow

Tests run on Linux and macOS (both need a C compiler for the
integration tests which compile and run generated C). Windows only
builds the Go binary since the generated C uses POSIX APIs.

All platforms produce a ccforth build artifact.

Author: ncw

.github/workflows/ci.yml

@@ -0,0 +1,51 @@
+name: CI
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.25"
+
+      - name: Run tests
+        run: go test ./...
+
+      - name: Build binary
+        run: go build -o ccforth ./cmd/ccforth
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: ccforth-${{ matrix.os }}
+          path: ccforth
+
+  build-windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.25"
+
+      - name: Build binary
+        run: go build -o ccforth.exe ./cmd/ccforth
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: ccforth-windows-latest
+          path: ccforth.exe