test(subagent-sdk): keyless Loader-composition e2e across the SDK wire

A test-only cordis.yml under examples/jsonrpc-agent boots the headless app
through the Loader; a scripted model delegates once to the SDK backend, whose
child is a COMPLETE second harness runtime (own cordis.yml, jsonrpc serving
surface, scripted cwd-echo model, own JSONL persistence). Asserts the parent
tool result AND the child's own persisted transcript both carry the parent
session's workspace cwd; child launch resolves through the shared
example-launch resolver so src/lib modes both hold.

examples/jsonrpc-agent/tests/fixtures/subagent/subagent-sdk/driver.ts

@@ -0,0 +1,15 @@
+#!/usr/bin/env node
+/** Test driver: one delegation turn through a headless Loader composition. */
+
+import { boot, resolveConfigPath } from '@deepseek-ai/dsh-app-boot'
+import { runOneShot } from '@deepseek-ai/dsh-cli-demo/src/cli.ts'
+
+const configPath = process.argv[2]
+if (configPath === undefined) throw new Error('sdk-subagent cwd driver requires a config path')
+
+const ctx = await boot('sdk-subagent-cwd-e2e', resolveConfigPath(configPath, undefined))
+try {
+  await runOneShot(ctx, { task: 'delegate' })
+} finally {
+  await ctx.fiber.dispose()
+}