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.

knip.json

@@ -15,6 +15,7 @@
         "headless-agent/tests/fixtures/time-context-mock-llm.ts",
         "tui-agent/tests/fixtures/tui-scripted-llm.ts",
         "acp-agent/tests/fixtures/subagent/subagent-acp/mock-delegating-llm.ts",
+        "acp-agent/tests/fixtures/subagent/subagent-acp/driver.ts",
         "*/tests/**/*.e2e.ts",
         "*/tests/**/*.snapshot.ts"
       ],