[FEATURE]: Bring subagents back to the session sidebar (port of #14043 / #4865 / #46109) #57097

What

Bring back a persistent sidebar list of a session's subagent child sessions, clickable to open, in both the TUI and the desktop/web app.

Why

V1 had working implementations of this and lost them in the V1→V2 move rather than on merit. On 2026-10-10 the following were all closed with the same automatic message ("This branch has been upgraded to V2. We're no longer taking PRs for V1. If you think this is still relevant, please port it over and open it for V2."):

• feat(web): Show subagents under parent session, allow intuitive navigation between. #14043 (web/desktop) — subagents shown as a collapsible tree under their parent session in the session sidebar, click to navigate to parent or child, subagent icon on the child.
• feat: add subagents sidebar with clickable navigation and parent keybind #4865 (TUI) — subagents sidebar with clickable navigation and a go-to-parent keybind.
• feat(tui): list active and recent subagents in the sidebar #46109 (TUI) — active subagents plus the last 10 finished ones, per-row live activity, collapsible, collapsed state in KV. Superseded feat: add subagents sidebar with clickable navigation and parent keybind #4865 and folded in [FEATURE]: Live Subagents sidebar section in the TUI #41249's suggestions.
• feat(tui): show subagent status in sidebar #36042 (TUI) — thinner status-only variant, closed earlier during automated cleanup.

#41249, the original request for this, is still open. No maintainer comment anywhere explains a design reason to drop it.

What exists in V2 today

• TUI: a composer "Subagents" tab, plus timeline cards.
• Desktop/web: subagent rows in the timeline, the running list in the session header, and subagent transcripts.

All of these are transient or scoped to one view. None gives a standing list of the session's children that you can click through, which is what makes an orchestration session navigable.

What I am asking for

A sidebar section, both surfaces, scoped to the session being viewed:

• Direct child sessions, newest activity first.
• Running state read from the child session's own status, not the parent's task tool part. The parent part reads completed as soon as a background child spawns, so filtering on it leaves the list empty (this bit the feat(tui): list active and recent subagents in the sidebar #46109 author in production).
• Click a row to open it: a tab where tabs are enabled, otherwise a route change.
• Collapsible, with the collapsed state persisted.
• Safe at scale: cap the list or default to active-only, so a session with hundreds of children stays readable.

The TUI side is small now that sidebar.content accepts plugin slots; the desktop/web side maps onto the side region's panels.

I have a working local TUI plugin that does the first point against 2.0.32 and can open it as a PR for the TUI half if that is wanted.