refactor: rename the backend to dsh-subagent-dsh-sdk

The group's convention is package suffix == provider default
(subagent-acp/'acp', subagent-spawn/'spawn', subagent-fork/'fork'), and the
provider default became dsh-sdk in the last review round — so the package
follows: @deepseek-ai/dsh-subagent-dsh-sdk at
packages/subagent/subagent-dsh-sdk, plugin name subagent-dsh-sdk,
diagnostics prefixed subagent-dsh-sdk:. The dsh echo has precedent
(dsh-llm-deepseek). Directory, fixture path, knip/tsconfig/examples
registrations, catalogs, READMEs (en+zh), and the Agent Note follow; the
sdk-client dispose ladder moves to its own module (src/dispose.ts) with the
deterministic FakeChild tier tests restored alongside it.

packages/subagent/subagent/tests/out-of-process.spec.ts

@@ -4,7 +4,7 @@
  * under their never-reject and idempotence contracts.
  */
 
-import { chmodSync, mkdtempSync, rmSync } from 'node:fs'
+import { chmodSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs'
 import { tmpdir } from 'node:os'
 import { join, relative, resolve } from 'node:path'
 import { describe, expect, it, vi } from 'vitest'
@@ -43,6 +43,17 @@ describe('child cwd resolution', () => {
     expect(() => assertUsableCwd('p', 'config cwd', join(tmpdir(), 'dsh-no-such-dir-xyz'))).toThrow('not an accessible directory')
   })
 
+  it('rejects an existing path that is a file, not a directory', () => {
+    const tmp = mkdtempSync(join(tmpdir(), 'oop-file-'))
+    const file = join(tmp, 'plain.txt')
+    try {
+      writeFileSync(file, 'not a dir\n')
+      expect(() => assertUsableCwd('p', 'config cwd', file)).toThrow('not an accessible directory')
+    } finally {
+      rmSync(tmp, { recursive: true, force: true })
+    }
+  })
+
   // Windows ACLs do not expose the POSIX directory search-bit state this fixture creates.
   it.skipIf(process.platform === 'win32')('rejects a directory without search permission', () => {
     // statSync().isDirectory() is true for a mode-600 directory, but a