tui: subagent tabs not restored after Ctrl+C restart until running subagent completes

Summary

After closing opencode2 with Ctrl+C while a subagent (background or foreground) is still running and then reopening it, the entire shell/subagent tab list is empty on reopen — neither the running subagent nor previously-completed (older) subagents appear. The moment the running subagent completes, the full list repopulates at once (older completed subagents + the now-completed one). The tab list is not restored on restart until the in-flight subagent finishes. Started after a recent next-channel update.

Environment

• opencode version: 0.0.0-next-16234 (opencode2, next channel; binary /home/scan/.bun/bin/opencode2 → @opencode-ai/cli, bun install)
• OS: Linux 7.0.0-28-generic (#28~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC) x86_64
• Terminal: ghostty (TERM=xterm-256color, TERM_PROGRAM unset)
• Shell: /bin/zsh
• Install/channel: next (bun-installed @opencode-ai/cli)
• Active plugins: gsd-core.js, herdr-agent-state.js, opencode-models-discovery@latest

Reproduction

1. Launch opencode2 in a workspace and run a few subagents so there are completed (older) subagents in the subagent tab list.
2. Trigger another subagent (background or foreground) that runs long enough to inspect mid-flight.
3. While that subagent is still running, press Ctrl+C to close opencode2.
4. Reopen opencode2 in the same workspace.
5. Navigate down to the shell and subagent tabs.
6. Observe: the tab list is empty — neither the running subagent nor the older completed subagents appear.
7. Wait for the running subagent to finish, then check the tabs again — the full list repopulates at once (older completed subagents + the now-completed one).

Expected Behavior

The subagent tab list should be restored on reopen, including previously-completed subagents and any subagent that is still running, so the user can see in-flight and historical subagents immediately after a restart.

Actual Behavior

The entire subagent tab list is empty after a Ctrl+C restart while a subagent is running. Nothing appears — not the running subagent, not older completed subagents — until the running subagent completes, at which point the whole list (old completed + the just-completed) repopulates together. The user loses visibility into all subagents (historical and in-flight) after a restart until the in-flight one finishes.

Additional Context

• Affects both background and foreground subagents (per reporter).
• The subagent process survives the Ctrl+C and completes later — the bug is that the tab list is not restored on reopen, not that the subagent is killed.
• Older completed subagents also vanish from the list on reopen; they reappear together with the running one's tab once it completes.
• Started after a recent next-channel update (reporter noticed it recently; current build 0.0.0-next-16234).
• Consistently reproducible since the update.
• Workarounds: none known; the full list repopulates only after the running subagent completes.