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_unix.go

@@ -0,0 +1,27 @@
+//go:build !windows
+
+package executor
+
+import (
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+// prepareNohup applies the Unix half of `nohup ... &`: the child's stdin is
+// closed, stdout and stderr are appended to logFile ("> output.log 2>&1") and,
+// with detach, the child becomes a session leader with no controlling terminal
+// — which also shields it from terminal-generated SIGHUP/SIGINT/SIGQUIT.
+//
+// Go's os/exec cannot reset a child's signal dispositions before exec (there is
+// no hook between fork and exec), so "ignore SIGHUP" itself is only provided by
+// the detached session, not by a signal handler.
+func prepareNohup(cmd *exec.Cmd, logFile *os.File, detach bool) {
+	// A nil stdin hands the child /dev/null instead of the terminal.
+	cmd.Stdout = logFile
+	cmd.Stderr = logFile
+
+	if detach {
+		cmd.SysProcAttr = &syscall.SysProcAttr{Setsid: true}
+	}
+}