TUI(collab): Persistent "Agents" sidepanel (active threads overview + quick actions)

Context

A recent collab TUI commit added agent selection (PR https://github.com/openai/codex/pull/9690). After you enter a thread, there’s no persistent view of other agents/threads that are still running, blocked, or completed. This creates lots of context switching back to the picker and scales poorly as the number of agents grows.

Request

Please add an optional persistent “Agents” sidepanel (toggleable; or auto-enabled when >1 agent exists) that remains visible while you’re in the main chat view.

MVP sidepanel contents

• Agent/thread list with state (queued/running/waiting/done) and last-activity timestamp.
• One-line preview per agent: last summary line (preferred) or last output line.
• Badges for: approvals pending, tool/user-input pending, error/failed, archived.
• Focus action: Enter/click jumps to that agent thread without going through a separate picker.

Nice-to-have quick actions (row-level)

• Queue follow-up prompt without interrupting (send_input interrupt=false).
• Interrupt + enqueue next prompt in one action (turn/interrupt then queued prompt).
• Close agent / dismiss from overview.

Acceptance criteria

• Sidepanel can be toggled on/off and does not break layout on small terminals (e.g., 80x24).
• Sidepanel updates in real time as agent output/status changes.
• Operator can jump between agents quickly without losing main-thread context.
• Pending approvals/tool requests are visible at a glance for the relevant thread.

Notes

This should be derivable from existing thread metadata + event stream (thread/*, turn/*, and item/* events; plus thread/list + thread/read(includeTurns=true) for hydration where needed).

Screenshots Reference

Image 1 (CodeMachine): Persistent left “workflow/agents” pane with a stable selection cursor; right pane shows the selected agent’s latest output without leaving the main view.
<img width="770" height="279" alt="Image" src="https://github.com/user-attachments/assets/25d22eb5-f778-493b-b3e2-08c8dc8e7453" />

Image 2 (CodeMachine): Same two-pane layout under load: multiple sub-agents running in parallel while the right pane stays pinned to one agent’s commands/logs (no repeated “go back to picker” workflow).
<img width="792" height="318" alt="Image" src="https://github.com/user-attachments/assets/ffbde9b1-1809-4acc-b231-28591877b8e4" />

Image 3 (Custom Supervisor TUI): Overview mode: left pane lists all agents with live status; right pane provides a compact per-agent summary + system log context while work continues.
<img width="1440" height="821" alt="Image" src="https://github.com/user-attachments/assets/7e87ea16-d9ff-43a6-8a5b-1e794af40883" />

Image 4 (Custom Supervisor TUI): Inspect mode: select an agent from the left list and read full scrollback/history in the right pane; agent switching stays O(1) (no context switch out of the main chat).
<img width="1440" height="823" alt="Image" src="https://github.com/user-attachments/assets/20d62aac-1f00-419a-b784-ebf0c9af9658" />

Related: openai/codex#2604 (Subagent Support)

Additional information

Search notes: I looked for existing issues around “agent picker”, “agents sidebar/sidepanel”, and “collab TUI”, but didn’t find a focused request for a persistent multi-agent overview pane. This is meant to complement the broader subagent discussion in #2604.

The UI in CodeMachine could be a good reference.