[feat] Achievement system — surface badges from game_state.json

[kody-w]

Description

game_state.json has achievements (first_visitor, first_trade, first_battle, world_host). Surface them as unlockable badges in a panel.

Tasks

• Achievement panel UI (press J or button in top bar)
• Badge icons for each achievement
• Toast notification on unlock
• Persistence in localStorage
• Echo: achievements affect agent reflexes (agents react to accomplished players)

Echo Level

L6 — persistent accumulation across sessions.