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.

examples/jsonrpc-agent/tests/fixtures/subagent/subagent-dsh-sdk/cordis.yml

@@ -14,8 +14,8 @@
 
 # providerName is omitted: the composition exercises the shipped default
 # (`dsh-sdk`) through the real Loader.
-- id: subagent-sdk
-  name: '@deepseek-ai/dsh-subagent-sdk'
+- id: subagent-dsh-sdk
+  name: '@deepseek-ai/dsh-subagent-dsh-sdk'
   config:
     command: !!js process.env.DSH_TEST_CHILD_COMMAND
     args: !!js JSON.parse(process.env.DSH_TEST_CHILD_ARGS ?? '[]')