ACP session creation ignores the default agent's assigned model

Summary

When using an ACP client (e.g. Zed) against opencode acp, new sessions are created with a model resolved from selectDefaultModel() (config model, last-used, or first provider model) — the default agent's assigned model is never consulted.

Impact

Configurations that assign per-agent models (e.g. oh-my-openagent style agent definitions where each agent declares model) start ACP sessions with the wrong model. The server-side resolution input.model ?? agent.model ?? currentModel (packages/opencode/src/session/prompt.ts) is bypassed because ACP always passes an explicit model to session.create/prompt.

Expected behavior

When the default agent (first primary, non-hidden agent) declares a model that resolves to a discovered provider model, new ACP sessions should default to that model. Client-explicit model selection must still win.