Expand description
Host-subprocess turn spawn (spawn_turn) for SubprocessController.
Runs the agent CLI as a std::process child (via tokio::process),
writes the turn message to its stdin, and drives two background tasks:
a stdout/stderr reader pair and a process-waiter that finalizes status
and drains the queued-message backlog. See container_spawn for the
Docker-exec analog used by container agents.
spawn_turn is one continuous, non-trivially-ordered state machine and
is moved here WHOLE rather than decomposed further.