run --format json events don't say which model produced them

Description

opencode run --format json emits step_start / step_finish events with tokens and cost, but no model. Nothing in the stream says which model produced a step, so a headless consumer can't attribute usage or cost to a model.

The data isn't missing from opencode — only from the stream. opencode export <sessionID> has providerID / modelID on every assistant message, and the default (human) format already prints the model header. It just never reaches the JSON path.

This matters for multi-model sessions: switching models mid-session, or subagents running a different model than the parent, are unattributable today. Recovering it needs a second opencode export call after the run, which doesn't work for a live consumer.

Event keys, same across three runs: cost, id, messageID, reason, sessionID, tokens, type.

OpenCode version

1.14.28

Steps to reproduce

1. opencode run --format json -m opencode/ling-3.0-flash-free "Reply with exactly: OK"
2. Continue that session with a different model: opencode run --format json --session <sessionID> -m opencode/mimo-v2.5-free "Reply with exactly: SECOND"
3. The step_finish events from both turns are indistinguishable:

{"id":"prt_…","reason":"stop","messageID":"msg_fcdd0d049001VruQNZZOX4Z9LQ","sessionID":"ses_032300dcdffeods08f6tn9QMAy","type":"step-finish","tokens":{"total":21335,"input":20277,"output":4,"reasoning":30,"cache":{"write":0,"read":1024}},"cost":0}

4. opencode export <sessionID> on that same session does distinguish them:

msg_fcdcff80c001Cp | opencode / ling-3.0-flash-free
msg_fcdd0386f001gQ | opencode / ling-3.0-flash-free
msg_fcdd0d049001Vr | opencode / mimo-v2.5-free

Operating System

macOS 26.5

Terminal

VS Code integrated terminal

Start at the entry point handling opencode run --format json and trace serialization for step_start and step_finish; compare it with the model fields shown by opencode export <sessionID>. Reproduce the two-model session from the issue and verify that live JSON events identify the provider and model for each step.