Add 10-second preparation countdown to !duel

[11k]

Summary

• Adds a 10-second preparation window before the duel phrase is revealed, giving participants time to get ready
• Removes redundant duel result message that was being sent to chat twice
• Updates all duel tests to account for the new async countdown flow

Flow

1. User types !duel user1 user2
2. Immediate message: "Get ready! The phrase to type will appear in 10 seconds..."
3. 10 seconds pass
4. Challenge message with phrase appears: "First to type '...' wins. You have 30 seconds. GO!"
5. 30-second typing window and resolution (unchanged)

Test plan

• All 196 tests passing
• Manually verify preparation message appears immediately
• Manually verify phrase appears ~10s later
• Verify 30s typing window works after phrase reveal
• Verify timeout works (10s prep + 30s typing)
• Verify concurrent duel rejection works during prep period