feat(sdk): TypeScript SDK client + shared wire protocol + SDK subagent backend

- @deepseek-ai/dsh-sdk-protocol: extract the line transport from dsh-jsonrpc
  and name the request/result/notification wire types both ends share; error
  responses preserve wire code/data via JsonRpcResponseError.
- @deepseek-ai/dsh-sdk-client: TypeScript twin of the Python SDK — spawns the
  dsh-jsonrpc-agent runtime as a subprocess, drives stdio JSON-RPC turns
  (DeepSeekHarness high-level API + HarnessClient protocol client), scopes
  notifications to session trees client-side, and reaps the child through the
  shared subprocess dispose ladder.
- @deepseek-ai/dsh-subagent-sdk: out-of-process subagent backend driving a
  child harness runtime through the TS SDK; shares cwd resolution with
  subagent-acp via new dsh-subagent-subprocess cwd helpers.
- Keyless unit suites drive real subprocesses (scripted fake runtime peer);
  100% per-file coverage on all touched packages.

tsconfig.host.json

@@ -142,6 +142,7 @@
     { "path": "./packages/subagent/subagent-spawn" },
     { "path": "./packages/subagent/subagent-fork" },
     { "path": "./packages/subagent/subagent-acp" },
+    { "path": "./packages/subagent/subagent-sdk" },
     { "path": "./packages/tasks/tasks" },
     { "path": "./packages/tasks/tasks-local" },
     { "path": "./packages/tasks/tool-tasks" },
@@ -159,7 +160,9 @@
     { "path": "./packages/mcp/mcp-client" },
     { "path": "./packages/host/apiproxy" },
     { "path": "./packages/host/webserver" },
+    { "path": "./packages/sdk/sdk-client" },
     { "path": "./packages/sdk/helper" },
+    { "path": "./packages/sdk/sdk-protocol" },
     { "path": "./packages/sdk/scripts" },
     { "path": "./packages/sdk/create-sdk" },
     { "path": "./packages/sdk/telemetry" },