Fixes #11598

Summary

Adds a new slash command "/edit-last" that opens the last assistant message in an external editor (vim/nvim/code etc.) for editing.

What does this PR do?

When users type "/edit-last" or select it from the command dialog, the last assistant message will be opened in the external editor configured via VISUAL or EDITOR environment variable.

Changes

• Added new command "Open last assistant message in editor" in session/index.tsx
• Command is only visible when VISUAL or EDITOR environment variable is set
• After editing and saving (:wq in vim), the edited content is loaded into the prompt input
• If no changes are made (:q!), the original message remains unchanged
• Users must press Enter to send the edited message

How did you verify your code works?

• Typechecked with bun run typecheck - all 12 packages passed
• Manual testing scenarios:
  1. Without EDITOR set: command does not appear in command palette
  2. With EDITOR=vim: /edit-last opens vim with last assistant message
  3. After :wq in vim: edited content appears in input prompt
  4. After :q! in vim: "No changes made" toast shown, input unchanged

Requirements

Users need to set the EDITOR or VISUAL environment variable:

export EDITOR=vim    # or nvim, nano, code, etc.
export VISUAL=nvim   # takes precedence over EDITOR