Chat: add starter prompt suggestions on empty chat

[BunsDev]

Description

When the chat view is empty (no messages yet), show clickable starter prompt cards/chips to help users get started quickly.

Examples

Suggested starters:

• "Explain this codebase"
• "Find and fix bugs"
• "Write tests for the open file"
• "Refactor for readability"
• "What does this function do?"

Behavior

• Show on the chat home screen below the hero/tagline
• Clicking a starter sends it as a message immediately
• Disappear once a conversation starts
• Should respect the current agent mode (Ask/Agent/Plan)
• Style: subtle cards/chips matching the current theme

Files

• components/chat-home.tsx — main implementation
• components/chat/message-list.tsx — hide starters once messages exist

[BunsDev]

Closing this because the empty-chat starter suggestions are already present in the app. The current implementation defines suggestion cards in components/chat-home.tsx and renders them in the empty chat state, including the scaffold/debug/review-style prompts mentioned in the issue.