ci: GitHub Actions 워크플로우 추가

Go 빌드 + 테스트 자동화 (push/PR on main)

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

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      # a301_server depends on tolchain via go.mod replace directive.
+      # Clone tolchain into ../tolchain so the replace path resolves.
+      - name: Checkout tolchain dependency
+        uses: actions/checkout@v4
+        with:
+          repository: ${{ github.repository_owner }}/tolchain
+          path: ../tolchain
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.25'
+
+      - name: Build
+        run: go build ./...
+
+      - name: Test
+        run: go test ./... -v