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/index.ts

@@ -0,0 +1,44 @@
+/**
+ * Shared no-shell `execFile` runner for host-native OS integrations (the
+ * native directory chooser, the open-with-default-application hand-off):
+ * utf8 stdio capture, abort propagation, Windows console hide. A library,
+ * not a plugin — no ctx, no state, no events.
+ * @module @deepseek-ai/dsh-native-command
+ */
+
+import { execFile } from 'node:child_process'
+
+/** Testable command boundary; native implementations never invoke a shell. */
+export type NativeCommandRunner = (
+  command: string,
+  args: readonly string[],
+  signal: AbortSignal,
+) => Promise<{ stdout: string; stderr: string }>
+
+/**
+ * Run a host command with utf8 stdio, abort propagation, and Windows hide.
+ * @param command - executable path or PATH name.
+ * @param args - argv (never a shell string).
+ * @param signal - caller/connection lifetime; abort terminates the child.
+ * @returns captured stdout/stderr on exit 0.
+ */
+export const runNativeCommand: NativeCommandRunner = (command, args, signal) =>
+  new Promise((resolve, reject) => {
+    execFile(
+      command,
+      [...args],
+      { encoding: 'utf8', signal, windowsHide: true },
+      (error, stdout, stderr) => {
+        if (error !== null) {
+          const failure = Object.assign(new Error(error.message, { cause: error }), {
+            code: error.code,
+            stdout,
+            stderr,
+          })
+          reject(failure)
+          return
+        }
+        resolve({ stdout, stderr })
+      },
+    )
+  })

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 */