feat: add keybinding to switch to last session #9526

Description

Currently, switching between sessions requires opening the session list dialog (ctrl-x l), navigating to the desired session, and pressing Enter. This is cumbersome when frequently switching between two sessions.

Proposed Solution

Add a session_last keybinding (<leader>o / ctrl-x o) to quickly switch to the previously viewed session, similar to Emacs's C-x o for switching windows.

The feature should:

• Track the previous session ID in memory (ephemeral, resets on restart)
• Allow instant switching with a single keybind
• Only be available when a previous session exists