[FEATURE]: Live Subagents sidebar section in the TUI

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I built a live "Subagents" sidebar section as an external TUI plugin (npm: opencode-subagents-view, source: https://github.com/giovannic96/opencode-subagents-view). It shows direct child sessions for the current session, with a per-row status icon, live activity text (current tool/step), and idle-run reset behavior when a new run starts.

I think this fits better as a built-in feature than an external plugin, since it's TUI-level, uses live session events already part of the app, and users would expect it to be built in rather than installed separately.

Related discussion I found:

• #15223 (subagents view in the TUI)
• #28175 (live session status panel in sidebar)
• #36042 (existing PR adding a thinner version of this, count + up to 5 titles, no per-row status icon or live activity)

Note: this has been discussed before, see links above, I'm linking them for context, not filing a duplicate.

I have a complete, tested implementation adapted to match this repo's conventions (packages/tui/src/feature-plugins/sidebar/*, same order/id/state-access patterns as context.tsx/lsp.tsx/todo.tsx) ready to open as a PR once there's a green light on the direction.