Serializable reference to a child agent process.
Replaces raw PIDs in strategy state for checkpoint/thaw safety. Contains all information needed to re-spawn a child from its checkpoint on resume.
This is a top-level Composer concept (not HITL-specific) since any suspension reason requires serializable child tracking.
Summary
Types
@type phase() :: :spawning | :awaiting_result | nil
@type status() :: :running | :paused | :hibernated | :completed | :failed