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/executor/builtin.go

@@ -0,0 +1,37 @@
+package executor
+
+import (
+	"context"
+	"io"
+)
+
+// BuiltinRequest carries the caller's arguments and output streams to an
+// in-process command implementation.
+type BuiltinRequest struct {
+	Args   []string
+	Stdout io.Writer
+	Stderr io.Writer
+}
+
+// Builtin executes a command inside kopsc instead of spawning a child. It is
+// used where no portable system utility exists (for example du on Windows).
+type Builtin func(ctx context.Context, req BuiltinRequest) error
+
+var builtins = map[string]Builtin{
+	"disk.usage": runDiskUsage,
+}
+
+// LookupBuiltin returns the implementation registered under name.
+func LookupBuiltin(name string) (Builtin, bool) {
+	fn, ok := builtins[name]
+	return fn, ok
+}
+
+// BuiltinNames lists the registered builtin implementations.
+func BuiltinNames() []string {
+	names := make([]string, 0, len(builtins))
+	for name := range builtins {
+		names = append(names, name)
+	}
+	return names
+}