Add CI workflow for PRs and pushes to main

Runs build, tests (with race detector), and go vet.

https://claude.ai/code/session_015F9BYQoCy2P2zYZBuVeXHH

.github/workflows/ci.yml

@@ -0,0 +1,38 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  build-and-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 ./... -timeout 60s -race
+
+  vet:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+
+      - name: Vet
+        run: go vet ./...