feat: add suggested starter prompts

[Maws7140]

Summary

Adds optional agent-generated starter prompt chips for fresh empty chats.

This is split from quick prompts because it sends a hidden ACP prompt request and has separate product/behavior implications which is kind of sus so I thought to separate it.

How it works

• On an empty session, a hidden instruction is sent asking for up to 4 concise
  prompts (under 12 words each), one per line.
• The reply is parsed into chips and suppressed from the visible message
  list the user never sees the request or the raw response.
• Clicking a chip sends that prompt.
• Gated behind a "Suggest starter prompts" setting.

Key changes

• src/hooks/useAgentMessages.ts generateStarterPrompts(), the hidden
  instruction, response parsing, and once-per-session guard
• src/ui/InputArea.tsx / src/ui/ChatPanel.tsx render the chips on empty chats
• src/ui/SettingsTab.ts the toggle