Chore: cli backend order in Select Backend (C-c a s) should follow most recent used order

[tninja]

The order for selection should be, eg:

1. most recently used backend (current backend, Codex CLI)
2. secondly recently used backend (eg. Antigravity CLI)
3. thirdly recently used backend (eg. ClaudeCode)
   ...

It might require keep the order somewhere external (eg. after selection, write it out), and load when press C-c a s

externalized file location can be under same directory as the history file of ai-code-read-string