ci: 新增 beta 分支自动构建流水线

- 触发：push beta / PR 到 beta / 手动触发
- test-and-build：SPA 构建 + go vet/test/build + 三平台交叉编译
  （linux/amd64、linux/arm64、windows/amd64），版本号为
  {VERSION}-beta.{sha}，二进制上传为 artifact
- docker-beta：多架构构建并推送 ghcr.io/{owner}/mmtl:beta
  （PR 事件不推镜像，仅推送 beta / 手动触发时推）
- 与 main 的发布流隔离：不做版本递增、不打 release tag

.github/workflows/beta-build.yml

@@ -0,0 +1,133 @@
+# Beta 分支自动构建流水线
+#
+# 触发：push 到 beta 分支 / PR 到 beta / 手动触发。
+# 产出：
+#   1. 前端 + 后端编译验证（go vet / go test / go build）
+#   2. 多平台可执行二进制 artifact（linux/amd64、linux/arm64、windows/amd64）
+#   3. ghcr.io/{owner}/mmtl:beta 多架构 Docker 镜像（linux/amd64 + linux/arm64）
+#
+# 与 main 分支的发布流（Auto-docker-publish.yml）隔离：beta 不做版本递增、
+# 不打 release tag，只构建带 -beta 标识的产物供测试。
+
+name: Beta Build
+
+on:
+  push:
+    branches: [beta]
+  pull_request:
+    branches: [beta]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  packages: write
+
+env:
+  BETA_VERSION_PREFIX: beta
+
+jobs:
+  # ─────────────────────────────────────────────────────────────────────────────
+  # 1) 编译验证 + 多平台二进制产物
+  # ─────────────────────────────────────────────────────────────────────────────
+  test-and-build:
+    name: Test & build artifacts
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Resolve beta version
+        id: version
+        run: |
+          BASE_VERSION=$(cat VERSION 2>/dev/null || echo "0.0.0")
+          SHA_SHORT=${GITHUB_SHA:0:7}
+          echo "full_version=${BASE_VERSION}-beta.${SHA_SHORT}" >> "$GITHUB_OUTPUT"
+
+      # The binary embeds the SPA (web/dist) via go:embed, so dist must exist
+      # before the Go toolchain touches the web package.
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+          cache-dependency-path: web/package-lock.json
+      - name: Build SPA
+        working-directory: web
+        run: |
+          npm ci
+          npm run build
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.25'
+          cache: true
+
+      - name: go vet
+        run: go vet ./...
+
+      - name: go test
+        run: go test ./...
+
+      - name: go build (host)
+        run: go build ./...
+
+      # 多平台可执行文件（嵌入刚构建的 web/dist）
+      - name: Build linux/amd64
+        run: CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -trimpath -ldflags="-s -w -X main.version=${{ steps.version.outputs.full_version }}" -o dist/mmtl-beta-linux-amd64 ./cmd/server
+      - name: Build linux/arm64
+        run: CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build -trimpath -ldflags="-s -w -X main.version=${{ steps.version.outputs.full_version }}" -o dist/mmtl-beta-linux-arm64 ./cmd/server
+      - name: Build windows/amd64
+        run: CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build -trimpath -ldflags="-s -w -X main.version=${{ steps.version.outputs.full_version }}" -o dist/mmtl-beta-windows-amd64.exe ./cmd/server
+
+      - name: Upload artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: mmtl-beta-binaries
+          path: dist/*
+          if-no-files-found: error
+
+  # ─────────────────────────────────────────────────────────────────────────────
+  # 2) Beta Docker 镜像（ghcr.io/{owner}/mmtl:beta）
+  # ─────────────────────────────────────────────────────────────────────────────
+  docker-beta:
+    name: Build & push beta Docker image
+    needs: test-and-build
+    runs-on: ubuntu-latest
+    # PR 事件不推送镜像，仅 push beta / 手动触发时推送
+    if: github.event_name != 'pull_request'
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Resolve beta version
+        id: version
+        run: |
+          BASE_VERSION=$(cat VERSION 2>/dev/null || echo "0.0.0")
+          SHA_SHORT=${GITHUB_SHA:0:7}
+          echo "full_version=${BASE_VERSION}-beta.${SHA_SHORT}" >> "$GITHUB_OUTPUT"
+
+      - uses: docker/setup-qemu-action@v3
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build & push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          provenance: false
+          sbom: false
+          tags: ghcr.io/${{ github.repository_owner }}/mmtl:beta
+          labels: |
+            org.opencontainers.image.revision=${{ github.sha }}
+            org.opencontainers.image.source=${{ github.repository }}
+          build-args: |
+            VERSION=${{ steps.version.outputs.full_version }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max