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/platform/platform.go

@@ -21,7 +21,7 @@ type Info struct {
 	Arch       string   // runtime.GOARCH, e.g. "amd64", "arm64"
 	Distro     string   // lower-case os-release ID, empty when unknown
 	DistroLike []string // lower-case os-release ID_LIKE entries
-	Runtimes   []string // container runtimes found on PATH, e.g. "docker", "podman"
+	Runtimes   []string // container runtimes found on PATH, e.g. "docker"
 }
 
 // Probe gathers Info. Its dependencies are injectable so detection can be
@@ -41,7 +41,7 @@ func System() Probe {
 		GOARCH:            runtime.GOARCH,
 		ReadFile:          os.ReadFile,
 		LookPath:          exec.LookPath,
-		RuntimeCandidates: []string{"docker", "podman"},
+		RuntimeCandidates: []string{"docker"},
 	}
 }
 
@@ -59,7 +59,7 @@ func (p Probe) Detect() Info {
 
 	candidates := p.RuntimeCandidates
 	if candidates == nil {
-		candidates = []string{"docker", "podman"}
+		candidates = []string{"docker"}
 	}
 	if p.LookPath != nil {
 		for _, name := range candidates {

internal/platform/platform_test.go

@@ -77,18 +77,6 @@ func TestDetect(t *testing.T) {
 				Runtimes: []string{"docker"},
 			},
 		},
-		{
-			name:      "linux centos with podman and docker",
-			goos:      "linux",
-			goarch:    "arm64",
-			osRelease: centosStream9,
-			present:   []string{"docker", "podman"},
-			want: Info{
-				OS: "linux", Arch: "arm64",
-				Distro: "centos", DistroLike: []string{"rhel", "fedora"},
-				Runtimes: []string{"docker", "podman"},
-			},
-		},
 		{
 			name:         "linux without os-release",
 			goos:         "linux",
@@ -133,7 +121,7 @@ func TestDetect(t *testing.T) {
 					}
 					return "", errors.New("not found")
 				},
-				RuntimeCandidates: []string{"docker", "podman"},
+				RuntimeCandidates: []string{"docker"},
 			}
 
 			got := probe.Detect()