Nest subagent threads in the legacy sidebar

[florin-szilagyi]

Problem

Every subagent run lands in the legacy sidebar as its own top-level thread. Agent-heavy sessions spawn dozens of them. The thread list floods and the real conversations drown. Nothing marks these rows as children of the run that made them.

Proposal

Nest subagent threads under the parent thread that spawned them.

• Child rows render indented and slightly smaller, collapsed by default behind the parent's "N subagents" badge.
• Clicking the badge expands or collapses that parent's children.
• A subagent whose parent thread is gone (archived or deleted) stays flat at the top level, so nothing disappears from the list.
• Shift-select spans only visible rows, so a range select never pulls in hidden subagent threads.

There is a draft PR with the working implementation, a screenshot, and focused tests: #18655

Happy to adjust scope or approach.

[vaststudios]

I have working code for this, extended to threads that agents start with t3_thread_launch and create_threads. Both kinds nest under the thread that started them, and a fold toggle hides them. Subagents start folded, and a folded parent still shows the status of its hidden threads. The full proposal with screenshots and two open questions is in #21266, and the code is in #21267.

#18655 was closed here for missing approval, so I would like a maintainer call on direction and scope before it goes further. If nesting subagents through the existing lineage is acceptable but the new launchedByThreadId field is not, I can cut the PR down to that.