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/nohup_windows.go

@@ -0,0 +1,32 @@
+//go:build windows
+
+package executor
+
+import (
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+// prepareNohup applies the Windows half of `nohup ... &`: the child's stdin is
+// connected to NUL and its stdout and stderr are appended to logFile
+// ("> output.log 2>&1").
+//
+// Windows has no SIGHUP; what the shell's `nohup ... &` buys is independence
+// from the launching console, which DETACHED_PROCESS provides: the child gets
+// no console at all, so closing the terminal kopsc ran in does not close the
+// child either. With detach disabled the child shares kopsc's console.
+func prepareNohup(cmd *exec.Cmd, logFile *os.File, detach bool) {
+	cmd.Stdout = logFile
+	cmd.Stderr = logFile
+
+	if detach {
+		// DETACHED_PROCESS: start without a console, and do not inherit one.
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			CreationFlags: syscall.CREATE_NEW_PROCESS_GROUP | detachedProcess,
+		}
+	}
+}
+
+// DETACHED_PROCESS (0x00000008) is not exported by package syscall.
+const detachedProcess = 0x00000008