feat: 新增 disk usage 内置命令（跨平台 du 等价）

- 注册表新增 Variant.Builtin 内置动作，与 Argv 互斥并参与平台匹配
- disk.usage 支持位置参数与 --path/--path=，输出按大小降序并带合计
- Unix 用设备号实现 -x 不跨文件系统，Windows 降级为不跟随 reparse point
- list/help/explain 支持展示内置动作，补充单元与 CLI 测试、README

internal/registry/registry_test.go

@@ -15,7 +15,7 @@ func TestBuiltinIsValid(t *testing.T) {
 
 func TestBuiltinIDsStable(t *testing.T) {
 	reg := Builtin()
-	want := []string{"docker.clearsafe", "net.ports", "pkg.refreshindex"}
+	want := []string{"docker.clearsafe", "disk.usage", "net.ports", "pkg.refreshindex"}
 	if got := reg.IDs(); !reflect.DeepEqual(got, want) {
 		t.Fatalf("IDs() = %v, want %v", got, want)
 	}
@@ -132,6 +132,45 @@ func TestValidateErrors(t *testing.T) {
 	}
 }
 
+func TestValidateBuiltinVariants(t *testing.T) {
+	builtinOnly := plainCommand("a")
+	builtinOnly.Variants = []Variant{{Match: Match{OS: "linux"}, Builtin: "demo.action"}}
+
+	if err := (&Registry{Commands: []Command{builtinOnly}}).Validate(); err != nil {
+		t.Fatalf("builtin-only variant should be valid: %v", err)
+	}
+
+	both := plainCommand("a")
+	both.Variants = []Variant{{Match: Match{OS: "linux"}, Argv: []string{"x"}, Builtin: "demo.action"}}
+	err := (&Registry{Commands: []Command{both}}).Validate()
+	if err == nil || !strings.Contains(err.Error(), "mutually exclusive") {
+		t.Fatalf("err = %v, want mutual-exclusion error", err)
+	}
+}
+
+func TestDiskUsageIsBuiltinEverywhere(t *testing.T) {
+	reg := Builtin()
+	cmd, ok := reg.Lookup("disk.usage")
+	if !ok {
+		t.Fatal("disk.usage not found")
+	}
+	seen := map[string]bool{}
+	for _, variant := range cmd.Variants {
+		if variant.Builtin != "disk.usage" {
+			t.Errorf("variant for %s should use the disk.usage builtin, got %q", variant.Match.OS, variant.Builtin)
+		}
+		if len(variant.Argv) != 0 {
+			t.Errorf("builtin variant must not carry argv: %v", variant.Argv)
+		}
+		seen[variant.Match.OS] = true
+	}
+	for _, osName := range []string{"linux", "darwin", "windows"} {
+		if !seen[osName] {
+			t.Errorf("disk.usage lacks a %s variant", osName)
+		}
+	}
+}
+
 func TestMatchIsComparable(t *testing.T) {
 	a := Match{OS: "linux", Distro: "centos", DistroLike: []string{"rhel"}, Runtime: "docker"}
 	b := Match{OS: "linux", Distro: "centos", DistroLike: []string{"rhel"}, Runtime: "docker"}