[feature] : preview for session message

[jensenojs]

Currently, when selecting a subagent for preview, only the session title and session ID are visible, but the specific conversation content cannot be seen.

In fact, this issue is not limited to selecting subagents.

If it is possible to provide some preview of message rendering within an acceptable time delay, it would be very helpful.

[jensenojs]

Rely on the performance improvements from #393 to simplify the implementation, rendering only the first and last messages

tested on three, It seems that mini picker doesn't support this feature, so I didn't implement it first.

snack

fzf-lua

telescope

[sudo-tee]

This is a really good UX improvement. I really like the idea :)

[jensenojs]

I'll push this PR up in a bit so you can give it a try,