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/resolve/resolve.go

@@ -188,18 +188,16 @@ func Evaluate(m registry.Match, info platform.Info) (score int, matched bool, re
 }
 
 // runtimeRank orders runtimes by preference when two variants are otherwise
-// equally specific: podman beats docker because it is the rootless default on
-// CentOS-family hosts, and a variant with no runtime constraint ranks last.
+// equally specific: docker ranks first and a variant with no runtime
+// constraint ranks last.
 func runtimeRank(runtime string) int {
 	switch strings.ToLower(runtime) {
-	case "podman":
-		return 0
 	case "docker":
-		return 1
+		return 0
 	case "":
-		return 3
-	default:
 		return 2
+	default:
+		return 1
 	}
 }