feat(resume-picker): add expand/collapse prompt view with keys

What feature would you like to see?

Add expand/collapse prompt view in resume picker with ←/→ keys

Problem:

The resume picker shows only a truncated first user message for each session. When browsing sessions, there's no way to inspect the full prompt without resuming the session first. This makes it difficult to identify the right session when you have many with similar starting text.

Proposal:

In the picker, let Right (→) expand the full first user prompt for the selected session; Left (←) collapses it. Show the expanded prompt inline directly under the selected row as one or more wrapped lines.

Example:
Resume a previous session
Type to search
Updated Branch Conversation

42 seconds ago main ▼ (expanded)

Can you help me refactor the authentication
module to use JWT tokens instead of session
cookies? I need to maintain backwards...
5 minutes ago feature Fix the login bug
2 hours ago main Add new API endpoint

Benefits:

• Quickly preview full prompt without resuming
• Helps identify the correct session among similar ones
• Non-intrusive - collapsed by default, expand only when needed

Additional information

Feat Req Demo

I have a working implementation ready:

https://github.com/user-attachments/assets/880f6dd5-a436-4b4c-a01b-1d87f4585bf3