tui: subagent model not shown anywhere

Summary

In the TUI, the model used by a subagent is not displayed anywhere: neither in the subagent's block in the main thread, nor in the task/session list, nor in the subagent detail view.

Environment

• opencode version: 1.18.16 (opencode --version)
• OS: macOS Darwin 25.6.0 (arm64)
• Terminal: TERM=xterm-256color
• Shell: /bin/zsh
• Install/channel: release (1.18.16)
• Active plugins: none found (no opencode.json/opencode.jsonc in cwd or ~/.config/opencode/, no .opencode/plugin(s))

Reproduction

1. In a session, spawn a subagent (foreground or background).
2. Inspect the subagent's block in the main thread, the task/session list, and the subagent detail view.
3. Nowhere is the model the subagent runs with displayed — only its name/status.

Expected Behavior

The subagent's configured model should be visible wherever the subagent is shown (main thread block, task/session list, detail view).

Actual Behavior

The subagent model is not displayed anywhere. Expanding the subagent block does not reveal it.

Additional Context

Observed while orchestrating multiple parallel subagents from the build agent. Consistent/reproducible, not intermittent. No workarounds found.