tui: no live preview of running subagent's last message

Summary

In the TUI, while a subagent is running in the background, there is no live preview of the subagent's last message in the main agent thread — the step shows only its status/spinner.

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. Start a background subagent.
2. In the main agent view, watch the running subagent step.
3. No live preview of the subagent's last message appears while it is running.

Expected Behavior

While a subagent is running, the main agent thread should show a live preview of the subagent's latest message.

Actual Behavior

A running (background) subagent shows no preview of its last message in the main thread — only status/spinner.

Additional Context

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