fix(opencode): switch the OpenCode V2 session model along with the approved agent (#1569)

OpenCode stores the session agent and model independently, so approving a plan switched to Build while leaving Plan's model active when the two agents are configured differently. The V2 adapter now keeps the model from agent discovery and applies switchAgent followed by switchModel, matching OpenCode's own clients; a host that cannot apply both leaves the session alone, and failures still leave plan approval intact.