Behaviour for runtimes that own an agent's inner execution loop.
Native Condukt agents use Condukt.AgentRuntimes.Native, where
Condukt.Session drives ReqLLM turns and Condukt tool calls. Runtime modules
that implement this behaviour receive the user prompt, a Condukt-owned
context map, and per-run options. Durable guidance is passed through the
composed :system_prompt value, including project instructions when enabled.
They return either a final text response or an explicit result map.