ci: add GitHub Actions workflow running go build and go test

Uses go-version-file so CI picks up the go.mod toolchain (go1.26.4);
tests run with the race detector.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: kyleconroy

.github/workflows/ci.yml

@@ -0,0 +1,22 @@
+name: ci
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+      - name: Build
+        run: go build ./...
+      - name: Test
+        run: go test -race ./...