[FEATURE]: List active subagents in the sidebar #46110

Describe the enhancement you want to request

A collapsible "Subagents" section in the sidebar for the current session — click a subagent to open its child session, like the other sidebar lists.

@franlol's #4865 already implements the base of this and is open. One thing that surfaces once you run it at scale: on an orchestration session that spawns hundreds of subagents, listing every one it ever spawned makes the sidebar unusable. Listing only the active subagents (in-flight/pending), deduped by child session id so a resumed task doesn't render twice, keeps it useful.

Filing this as the tracking issue for the feature since #4865 didn't have one. #46109 implements the active-only variant.