Messages duplicate on channel switch / WebSocket reconnect

[kagura-agent]

Bug

When switching channels or when the WebSocket reconnects, messages appear duplicated — identical pairs of user/assistant messages.

Root Cause

sendAllChannelHistory() sends historical messages using the same type: 'message' event as real-time new messages. The frontend App.tsx appends every message event to state without dedup.

On reconnect, history is re-sent → messages double.

Fix Options

1. Frontend dedup: check message id before appending
2. Server: use a distinct channel_history batch event (not individual message events)
3. Frontend: clear channel messages on reconnect before loading history

Option 2 is cleanest — separates history from live messages.

Steps to Reproduce

1. Open Workshop UI
2. Send some messages in a channel
3. Switch to another channel and back
4. Messages appear duplicated