fix(subagent-acp): repair the composition fixture for the merged depth budget

Master landed the tool-subagent depth budget while this branch was in
flight: the numeric default maxDepth now fails the mount against the
ACP provider's depthLimit: false, so the composition fixture must state
maxDepth: 'provider-managed' — the documented opt-out for a provider
whose recursion budget lives in the child harness. The fixture also
moves off the retired stdio-demo REPL onto the current app-boot driver
pattern (runLoaderSmoke + one-shot cli), and the split isDirectory
statements gain the file-not-a-directory case the single-expression
form used to cover implicitly.

examples/acp-agent/tests/fixtures/subagent/subagent-acp/cordis.yml

@@ -26,12 +26,16 @@
   config:
     provider: acp
     toolName: subagent
+    # ACP advertises no depthLimit: the child harness owns its own recursion
+    # budget, so the local numeric default cannot apply here.
+    maxDepth: 'provider-managed'
 
-- id: stdio-agent
-  name: '@deepseek-ai/dsh-stdio-demo'
+- id: cli-agent
+  name: '@deepseek-ai/dsh-cli-demo'
   config:
     provider: mock
     model: mock-delegate
-    welcome: 'acp subagent cwd e2e ready.'
+    persona: 'Test ACP subagent cwd inheritance.'
     persistenceRoot: './.sessions'
+    persistenceCompression: 'none'
     workspaceContext: false

examples/acp-agent/tests/fixtures/subagent/subagent-acp/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('acp-subagent cwd driver requires a config path')
+
+const ctx = await boot('acp-subagent-cwd-e2e', resolveConfigPath(configPath, undefined))
+try {
+  await runOneShot(ctx, { task: 'delegate' })
+} finally {
+  await ctx.fiber.dispose()
+}