Agents Window layout should be restored per session

When switching between sessions in the Agents Window, the editor layout is currently shared globally across the app instead of being restored per session.

For example, a session that uses a diff may have a split layout with a secondary pane. Switching from that session to a no-workspace, chat-only session preserves the split and leaves half of the window occupied by an empty pane. That pane cannot be used directly without opening something through the Command Palette.

This becomes increasingly disruptive as sessions vary between chat-only work and sessions using terminals, diffs, or browsers.

## Steps to reproduce

1. Open an agent session and arrange the window so a diff or another resource occupies a separate pane.
2. Switch to a chat-only session, such as a no-workspace session.
3. Observe that the previous session's split layout remains, leaving an empty pane.

## Expected behavior

Each agent session restores its own layout, so chat-only sessions can use the full window while sessions with terminals, diffs, browsers, or other resources retain their own arrangement.

## Actual behavior

The layout is global across sessions, so switching between sessions with different resource needs can leave large empty panes that are not directly useful.

Reported with two screen recordings demonstrating the behavior in Slack.