e2e: register factoryai-droid concurrency gate

Same latent no-op as cursor-cli: the workflow sets E2E_CONCURRENT_TEST_LIMIT=1 for factoryai-droid but the agent never called RegisterGate, so AcquireSlot did nothing and it ran at full parallelism. Register the gate (default 1) so the intended serialization is respected. Can re-parallelise later if it proves safe.