Add /history slash command tool to agent

[Miracle656]

Summary

Users can ask the agent for transfer history naturally, but a slash command (/history 10) would be faster. Add it as a tool the agent recognizes.

Files

• packages/agent/src/agent.ts
• packages/agent/src/txBuilder.ts (already has get_transfer_history)

Acceptance criteria

• Frontend recognizes /history and /history N
• Sends a structured prompt to the agent
• Agent responds with a formatted list (date, direction, asset, amount)