feat(tui): list active session subagents in the sidebar

[iceteaSA]

Issue for this PR

Closes #41249. Supersedes #4865 by @franlol (open).

Type of change

• New feature

Credit

The feature and the original implementation are @franlol's in #4865 — subagents in the sidebar, click to navigate. This is that, rebased onto dev, with one change that came out of using it: we ran an orchestration session with hundreds of subagents, and listing every one made the sidebar useless. So this lists only active subagents and dedupes by child session id, so a resumed task_id doesn't render twice. If @franlol revisits #4865, that filter is the part worth lifting. If theirs lands, I'll close this. @giovannic96 requested the same feature in #41249 after shipping it as an external plugin.

Parent navigation and the return-to-parent keybind #4865 also added are already on dev, so this is just the list.

What does this PR do?

Adds a collapsible "Subagents" section to the sidebar showing the session's active subagents; select one to open that child session. The derive helper sits in packages/core since packages/tui can't import opencode's private symbols.

Liveness comes from the child session's status, not the parent's task tool-part status. That distinction is load-bearing: a background dispatch completes the parent's tool part the instant the child spawns, so the parent part reads completed while the child runs for minutes. On my machine, of the last 300 task parts, 270 were completed and zero were running — including a subagent that was executing at the time. Filtering on the parent part yields an empty list forever and the section silently never renders. api.state.session.status(childID) is the correct signal, since idle sessions are absent from that map by design.

How did you verify your code works?

• packages/tui: 257 pass / 0 fail — covers derive, dedup, the active filter, and the background-completed-parent-with-busy-child case that the parent-status approach got wrong
• bun typecheck clean in packages/tui
• Confirmed rendering in a live session with a running background subagent

Screenshots / recordings

A "Subagents" list in the sidebar, active only, collapsible like the other lists.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Related PRs Found

#4865: feat: add subagents sidebar with clickable navigation and parent keybind

• feat: add subagents sidebar with clickable navigation and parent keybind #4865
• Why related: This is the original PR that this one supersedes. PR feat(tui): list active session subagents in the sidebar #46109 builds on the parent navigation concept from feat: add subagents sidebar with clickable navigation and parent keybind #4865 but changes the design to list only active subagents instead of every subagent ever spawned, addressing performance issues with long-running orchestrations.

#14043: feat(web): Show subagents under parent session, allow intuitive navigation between

• feat(web): Show subagents under parent session, allow intuitive navigation between. #14043
• Why related: Similar feature for the web interface—showing subagents under parent sessions and allowing navigation between them.

#25712: feat(tui): show subagent cost rollup in sidebar and task history

• feat(tui): show subagent cost rollup in sidebar and task history #25712
• Why related: Related sidebar enhancement for subagents, specifically showing cost information alongside the subagent list.