feat: add CI workflow for backend and frontend build, typecheck, and test

.github/workflows/ci.yml

+name: CI
+
+on:
+  pull_request:
+    types: [opened, synchronize, closed]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  backend:
+    name: Backend build and test
+    if: github.event.action != 'closed' || github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    env:
+      GOWORK: 'off'
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+
+      - name: Set up Go
+        uses: actions/setup-go@924ae3a1cded613372ab5595356fb5720e22ba16 # v6.5.0
+        with:
+          go-version-file: go.mod
+          cache-dependency-path: |
+            go.sum
+            relaykit/go.sum
+
+      - name: Create frontend embed placeholder
+        run: |
+          mkdir -p web/dist
+          touch web/dist/index.html
+
+      - name: Build root module
+        run: go build ./...
+
+      - name: Build relaykit independently
+        working-directory: relaykit
+        run: go build ./...
+
+      - name: Test root and relaykit modules
+        run: make test
+
+  frontend:
+    name: Frontend typecheck and test
+    if: github.event.action != 'closed' || github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    defaults:
+      run:
+        working-directory: web
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+
+      - name: Set up Bun
+        uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
+        with:
+          bun-version: '1.3.14'
+
+      - name: Install dependencies
+        run: bun install --frozen-lockfile
+
+      - name: Typecheck
+        run: bun run typecheck
+
+      - name: Test
+        run: bun test