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/requirements.txt

@@ -6,3 +6,5 @@ anthropic>=0.18.0
 python-dotenv>=1.0.0
 aiohttp>=3.8.0
 redis>=5.0.0
+loguru>=0.7.0
+tiktoken>=0.12.0