Allow an explicit bootstrap turn when native session instructions are unavailable

[taras]

Problem

Native agent-session launch prepares context as provider session/system/developer instructions before handing the durable session to the provider's native UI. V1 fails when a provider cannot establish that context without a model turn.

Some providers may only be able to place prepared context into conversation history by performing a prompt before handoff. Silently doing that would add model cost, produce an assistant response, mutate retained history, and change replay semantics.

Future behavior

Add an explicit, author-visible opt-in for a bootstrap prompt during native session preparation.

The eventual contract must preserve these constraints:

• no provider silently falls back from session instructions to a prompt;
• the bootstrap is an ordinary durable Agent prompt with its exact input, output, cost-visible turn, and terminal outcome retained;
• the native UI launches only after the bootstrap completes successfully;
• bootstrap failure or cancellation prevents handoff;
• replay restores a completed bootstrap without invoking the model again;
• partial replay follows the ordinary Prompt reconciliation contract;
• the generated assistant response is visible to the author/operator rather than discarded;
• providers capable of installing session instructions continue to do so without a bootstrap turn.

Syntax and provider capability negotiation remain to be designed with the native session-launch feature.