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/util/native-command/src/invariant.ts

@@ -0,0 +1,31 @@
+/**
+ * Package-owned invariant companion for `@deepseek-ai/dsh-native-command`.
+ * @module @deepseek-ai/dsh-native-command/invariant
+ */
+
+/* jscpd:ignore-start */
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-native-command'
+
+/** Cordis companion plugin name. */
+export const name = 'native-command-invariant'
+/** Service required before the companion can reserve package ownership. */
+export const inject = ['invariants']
+
+/**
+ * No runtime invariant: each run is one stateless child-process round trip
+ * with no owned event stream or mutable runtime data; behavior is enforced by
+ * unit tests.
+ */
+const install: InvariantInstaller = () => {}
+
+/**
+ * Register this package's invariant companion.
+ * @param ctx - Cordis context carrying the invariant service.
+ * @returns the installed registration's disposer after setup succeeds.
+ */
+export const apply = (ctx: Context): Promise<() => void> =>
+  Promise.resolve(ctx.invariants.register(PACKAGE_NAME, install))
+/* jscpd:ignore-end */