test(subagent-acp): cover inherited cwd through a Loader composition

The inheritance branch ran only under hand-mounted plugin tests, and
the with-key e2e always configures an explicit cwd. Add a test-only
cordis.yml (scripted delegating model + the scripted mock ACP child)
booting the stdio app through the real Loader with cwd omitted: the
child proves it ran in — and was announced — the parent session's
workspace, keylessly.

knip.json

@@ -12,6 +12,7 @@
         "echo-agent/src/*.ts",
         "headless-agent/tests/fixtures/cli-mock-llm.ts",
         "tui-agent/tests/fixtures/tui-scripted-llm.ts",
+        "acp-agent/tests/fixtures/subagent/subagent-acp/mock-delegating-llm.ts",
         "*/tests/**/*.e2e.ts",
         "*/tests/**/*.snapshot.ts"
       ],