Add streaming support and refactor Chat UI

- Add run_stream method to AgentCore for streaming output
- Add base_url parameter to LLM clients for OpenRouter support
- Add xbot module for new agent implementation
- Refactor Chat.vue into composable + components (ChatHeader, ChatMessage, ChatInput, ChatSidebar, ChatAgentSelector)
- Add ChatStream handler for SSE streaming in Go server
- Add UseXBot field to chat request

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agent/app/xbot/__init__.py

@@ -0,0 +1,17 @@
+"""XBot - 轻量级 Agent 框架（基于 nanobot 核心）"""
+
+from .loop import AgentLoop
+from .memory import MemoryConsolidator, MemoryStore
+from .session import Session, SessionManager
+from .adapter import XBotLLMAdapter
+from .agent import XBotAgent
+
+__all__ = [
+    "AgentLoop",
+    "MemoryConsolidator",
+    "MemoryStore",
+    "Session",
+    "SessionManager",
+    "XBotLLMAdapter",
+    "XBotAgent",
+]