[FEATURE]: v2 SDK: bind agent/model/variant options to queued prompts

Verification

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I am porting Kimaki to v2. I need to queue a prompt with its own agent, model, and variant without changing the work already running.

Proposed SDK API:

await client.session.prompt({
  sessionID,
  text: "Review the changes",
  delivery: "queue",
  agent: "review",
  model: {
    providerID: "anthropic",
    modelID: "claude-opus-4-6",
    variant: "high",
  },
})

Store these options with the durable input and apply them when it is delivered, not when it is queued. Omitted options keep the existing behavior.

Today, separate session.switchAgent() / session.switchModel() calls change session-wide selection before the queued prompt runs. Attaching options to the input would let Kimaki use the OpenCode queue directly instead of keeping a separate client-side queue to coordinate selection.

This is a proposed API capability, not a reproduced concurrency bug.