[spacedevin]

• CallModel takes an optional fifth argument, ModelRequest { toolChoice }; createHyperyModel sends it as
  tool_choice ("auto" stays the default). forceTool(name) builds the one-tool choice.
• runAgentLoop takes toolChoice: a ToolChoice for every round or a function of the round; the wrap-up call
  after maxRounds never forces one.
• AgentHost.childSystemPrompt may return a Promise; the runtime awaits it.
• The task tool passes run_in_background only when the model set it, so a profile's background applies.
• A child whose stop signal (its own or the parent's) is raised is recorded as "stopped" even when the stop
  surfaced as a thrown error, as it does natively.