Add agent_chat_stream Tauri command that:
- Accepts StreamChatRequest with agent_id, session_id, message
- Gets streaming receiver from kernel.send_message_stream()
- Spawns background task to emit Tauri events ("stream:chunk")
- Emits StreamChatEvent types (Delta, ToolStart, ToolEnd, Complete, Error)
- Includes session_id for frontend routing
Registered in lib.rs invoke_handler.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>