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.go

@@ -22,7 +22,11 @@ type Variant struct {
 	Match Match
 	Argv  []string // program followed by arguments
 	Shell bool     // run through the platform shell instead of exec'ing Argv[0]
-	Note  string   // human-readable rationale shown by `kopsc explain`
+	// Builtin names an in-process implementation registered with the executor.
+	// It is mutually exclusive with Argv and lets kopsc provide commands that
+	// have no equivalent system utility (for example on Windows).
+	Builtin string
+	Note    string // human-readable rationale shown by `kopsc explain`
 }
 
 // Command is a platform-neutral intent, e.g. "docker.clearsafe".
@@ -73,7 +77,13 @@ func (r *Registry) Validate() error {
 		}
 
 		for j, variant := range cmd.Variants {
-			if len(variant.Argv) == 0 || strings.TrimSpace(variant.Argv[0]) == "" {
+			if len(variant.Argv) == 0 && variant.Builtin == "" {
+				return fmt.Errorf("command %q variant #%d: empty argv and no builtin", cmd.ID, j)
+			}
+			if len(variant.Argv) > 0 && variant.Builtin != "" {
+				return fmt.Errorf("command %q variant #%d: argv and builtin are mutually exclusive", cmd.ID, j)
+			}
+			if len(variant.Argv) > 0 && strings.TrimSpace(variant.Argv[0]) == "" {
 				return fmt.Errorf("command %q variant #%d: empty argv", cmd.ID, j)
 			}
 			if err := variant.Match.validate(); err != nil {