Agent definition fields

Agent accepts a name, provider/model, instructions, tools, sub-agents, and runtime policy. Important fields include strategy, max_turns, max_tokens, temperature, timeout_seconds, output_type, guardrails, termination, handoffs, credentials, stateful, enable_planning, callbacks, and fallback.

Names must match ^[a-zA-Z_][a-zA-Z0-9_-]*$. Empty models represent inherited or external-agent behavior. The complete constructor and serialization semantics are maintained in api-reference.md and AgentConfigSerializer; use those sources when adding a newly supported field.