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/registry/registry.go

@@ -14,7 +14,7 @@ type Match struct {
 	Arch       string   // runtime.GOARCH
 	Distro     string   // exact os-release ID, e.g. "centos"
 	DistroLike []string // os-release ID_LIKE ancestry, e.g. "rhel"
-	Runtime    string   // container runtime required on PATH, e.g. "podman"
+	Runtime    string   // container runtime required on PATH, e.g. "docker"
 }
 
 // Variant is one concrete implementation of a command.
@@ -27,6 +27,10 @@ type Variant struct {
 	// have no equivalent system utility (for example on Windows).
 	Builtin string
 	Note    string // human-readable rationale shown by `kopsc explain`
+	// Runtime records which container runtime a builtin should drive when
+	// Argv cannot carry it (builtin variants have no argv). Resolution uses
+	// Match.Runtime; this field is only informational context for the builtin.
+	Runtime string
 }
 
 // Command is a platform-neutral intent, e.g. "docker.clearsafe".