feat(ops): U4 后端镜像瘦身 282MB→107MB (62%降幅)

- 移除 git/docker-cli/docker-cli-buildx (~100MB) 改为懒加载
- PipelineHandler 注入 ToolInstaller, runCheckout/runImageBuild 调 EnsureAvailable
- UpgradeTools 自动升级 DB 中 git/docker-cli preinstall→lazy
- 仅内置 linux-amd64 agent (~12MB节省), arm64/darwin 通过下载API
- kubectl 改用 install-kubectl.sh 下载 stripped 版本
- .dockerignore 排除 backend/kubectl 60MB 二进制
- Dockerfile.embed + backend/Dockerfile 同步更新
- 17/17 集成测试通过

backend/model/deploy_tool.go

@@ -191,29 +191,31 @@ echo "[Node.js] Installed: $(node --version) / npm $(npm --version)"
 `,
 			SortOrder: 4,
 		},
-		// ──── Git — 版本控制（预装）// Logan.Li ────
+		// ──── Git — 版本控制（懒加载, U4 镜像瘦身）// Logan.Li ────
 		{
-			Name:          "git",
-			DisplayName:   "Git",
-			Description:   "版本控制工具，流水线代码拉取使用",
-			Category:      "build",
-			InstallMethod: ToolPreinstall,
-			BinaryName:    "git",
-			InstallPath:   "/usr/bin/git",
-			CheckCommand:  "git --version 2>/dev/null",
-			SortOrder:     5,
+			Name:           "git",
+			DisplayName:    "Git",
+			Description:    "版本控制工具，流水线代码拉取使用（首次使用自动安装）",
+			Category:       "build",
+			InstallMethod:  ToolLazy,
+			BinaryName:     "git",
+			InstallPath:    "/usr/bin/git",
+			CheckCommand:   "git --version 2>/dev/null",
+			InstallScript: `apk add --no-cache git`,
+			SortOrder:      5,
 		},
-		// ──── Docker CLI — 容器构建（预装）// Logan.Li ────
+		// ──── Docker CLI — 容器构建（懒加载, U4 镜像瘦身）// Logan.Li ────
 		{
-			Name:          "docker-cli",
-			DisplayName:   "Docker CLI",
-			Description:   "Docker 命令行工具，流水线镜像构建/推送使用",
-			Category:      "deploy",
-			InstallMethod: ToolPreinstall,
-			BinaryName:    "docker",
-			InstallPath:   "/usr/bin/docker",
-			CheckCommand:  "docker --version 2>/dev/null",
-			SortOrder:     6,
+			Name:           "docker-cli",
+			DisplayName:    "Docker CLI",
+			Description:    "Docker 命令行工具，流水线镜像构建/推送使用（首次使用自动安装）",
+			Category:       "deploy",
+			InstallMethod:  ToolLazy,
+			BinaryName:     "docker",
+			InstallPath:    "/usr/bin/docker",
+			CheckCommand:   "docker --version 2>/dev/null",
+			InstallScript:  `apk add --no-cache docker-cli`,
+			SortOrder:      6,
 		},
 	}
 }