Chat history UI should match proposed design

Steps to reproduce

1. Open the AI sidepanel
2. Navigate to Chat History
3. Compare the layout to design in Figma

Current behavior

• Chat history still uses the old design with prominent gray retention message box (solved via gitlab-foss#81099 (moved))
• Delete buttons (trash icons) are always visible on every chat item

Expected behavior

Chat history should be updated to match the proposed design in the AI Side Panel Figma:

• Hide delete buttons by default, show only on hover or selection
• Adopt the cleaner spacing, typography, and visual hierarchy from proposed design
• Agent Avatars are added to the list
• Chat history is renamed to Threads