Support multiple independent chats in tiled / split view

What variant of Codex are you using?

Codex App

What feature would you like to see?

I would like Codex App to support displaying multiple independent, first-class chat threads simultaneously within the same window.

Each pane should contain a normal Codex conversation with its own context, composer, history, and agent execution.

This is specifically about multiple normal, persistent, independent conversations being visible at the same time — not /side, a subagent view, or simply tabs for switching between chats.

Ideally, users could:

• split the current workspace vertically or horizontally;
• open an existing or new chat in another pane;
• display 2, 3, or more chats simultaneously;
• resize individual panes;
• move chats between panes;
• maximize or close individual panes;
• preserve the layout per project/workspace.

This would be especially useful for parallel agent workflows, for example keeping one chat working on implementation, another discussing architecture, and another handling tests or debugging.

For example:

┌────────────────────┬────────────────────┬────────────────────┐
│       Chat A       │       Chat B       │       Chat C       │
│                    │                    │                    │
│   implementation   │    architecture    │  tests / research  │
│                    │                    │                    │
└────────────────────┴────────────────────┴────────────────────┘

### Additional information

Codex already supports working with multiple tasks and conversations, but switching between them means losing visual context.

With larger or ultrawide monitors, being able to keep several independent chats visible at once would make Codex much more effective as an agentic development workspace.

Example workflow:

- Chat A: implements a feature
- Chat B: reviews architecture and design decisions
- Chat C: runs tests, investigates failures, or performs research

The important distinction is that these should all be regular, independent Codex threads.

This is related to existing requests for multi-pane workspaces, such as #33301, but the core request here is narrower: allowing multiple independent Codex chat threads themselves to occupy panes simultaneously, e.g. `chat + chat + chat`.

Tabbed chats improve switching between conversations; this feature would improve simultaneous monitoring and interaction with multiple conversations.

No files, tests, or entry points are named in the issue. Start by locating the Codex App window or workspace layout and its existing conversation and tab handling, then clarify the smallest supported pane scope with maintainers. Done should cover multiple independent chats shown simultaneously, resizing and closing panes, and layout persistence if that remains in scope.