refactor(util): extract the shared no-shell native-command runner to dsh-native-command

master's toolcall-open extracted runNativeCommand inside apiproxy for the
openPath opener while the picker seam had moved the native chooser (its other
consumer) into directory-picker-native; after the merge the two packages each
carried a verbatim copy. The runner now lives in packages/util/native-command
(zero-dependency library, per the util-group contract) and both native
integrations depend on it.

packages/host/directory-picker-native/src/native-picker.ts

@@ -1,6 +1,6 @@
 /** Cross-platform native single-directory chooser behind the dialog backend's capability. */
 
-import { runNativeCommand, type NativeCommandRunner } from './native-command.ts'
+import { runNativeCommand, type NativeCommandRunner } from '@deepseek-ai/dsh-native-command'
 
 /** Testable command boundary; native implementations never invoke a shell. */
 export type DirectoryPickerRunner = NativeCommandRunner