feat(streaming): add Tauri streaming chat command

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>

desktop/src-tauri/src/lib.rs

@@ -1332,6 +1332,7 @@ pub fn run() {
             kernel_commands::agent_get,
             kernel_commands::agent_delete,
             kernel_commands::agent_chat,
+            kernel_commands::agent_chat_stream,
             // OpenFang commands (new naming)
             openfang_status,
             openfang_start,
@@ -1347,7 +1348,6 @@ pub fn run() {
             openfang_process_logs,
             openfang_version,
             // Health check commands
-            openfang_health_check,
             openfang_ping,
             // Backward-compatible aliases (OpenClaw naming)
             gateway_status,