Agent backend transports and provider-neutral contracts.
This crate owns the agent execution boundary used by Agentty: provider
model metadata, turn prompt payloads, CLI/app-server transports, and
channel contracts. [OneShotClient] presents isolated structured prompts
as one injectable operation, while [TurnContinuation] keeps transport
recovery state out of [TurnRequest]. The crate intentionally avoids
depending on the agentty application crate so provider-specific
dependencies compile in a leaf workspace member.