feat: 新增 proc nohup 后台运行、docker save/load 内置实现，移除 Podman 支持

- proc.nohup：后台运行命令并将 stdout/stderr 追加写入日志，
  Unix 下新会话、Windows 下 DETACHED_PROCESS 脱离终端
- docker.save：等价 docker save <镜像> -o <镜像名>.tar，文件名按
  仓库名+标签生成（alpine:3.20 → alpine_3.20.tar），传镜像 ID 时
  先解析出对应名称；docker.load 改为归档文件作位置参数
- 移除 Podman 支持，所有容器命令只走 docker

internal/executor/builtin.go

@@ -15,6 +15,10 @@ type BuiltinRequest struct {
 	Stdout io.Writer
 	Stderr io.Writer
 	Info   platform.Info
+	// Plan is the resolved variant and forwarded arguments. Builtins that
+	// drive an external runtime (for example docker save) read the runtime's
+	// program name and the passthrough arguments from it.
+	Plan Plan
 }
 
 // Builtin executes a command inside kopsc instead of spawning a child. It is
@@ -22,9 +26,12 @@ type BuiltinRequest struct {
 type Builtin func(ctx context.Context, req BuiltinRequest) error
 
 var builtins = map[string]Builtin{
-	"disk.usage": runDiskUsage,
-	"diagnose":   runDiagnose,
-	"net.pids":   runNetPIDs,
+	"disk.usage":  runDiskUsage,
+	"diagnose":    runDiagnose,
+	"docker.load": runDockerLoad,
+	"docker.save": runDockerSave,
+	"net.pids":    runNetPIDs,
+	"proc.nohup":  runNohup,
 }
 
 // runDiagnose bridges the registry builtin to the diagnose package.