feat: add AIO sandbox provider and auto title generation (#1)

- Add AioSandboxProvider for Docker-based sandbox execution with
  configurable container lifecycle, volume mounts, and port management
- Add TitleMiddleware to auto-generate thread titles after first
  user-assistant exchange using LLM
- Add Claude Code documentation (CLAUDE.md, AGENTS.md)
- Extend SandboxConfig with Docker-specific options (image, port, mounts)
- Fix hardcoded mount path to use expanduser
- Add agent-sandbox and dotenv dependencies

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/agents/lead_agent/agent.py

@@ -1,15 +1,18 @@
 from langchain.agents import create_agent
 
 from src.agents.lead_agent.prompt import apply_prompt_template
+from src.agents.middlewares.title_middleware import TitleMiddleware
 from src.agents.thread_state import ThreadState
 from src.models import create_chat_model
 from src.sandbox.middleware import SandboxMiddleware
 from src.tools import get_available_tools
 
+middlewares = [SandboxMiddleware(), TitleMiddleware()]
+
 lead_agent = create_agent(
     model=create_chat_model(thinking_enabled=True),
     tools=get_available_tools(),
-    middleware=[SandboxMiddleware()],
+    middleware=middlewares,
     system_prompt=apply_prompt_template(),
     state_schema=ThreadState,
 )

backend/src/agents/lead_agent/prompt.py

@@ -1,6 +1,7 @@
+import os
 from datetime import datetime
 
-MOUNT_POINT = "/Users/henry/mnt"
+MOUNT_POINT = os.path.expanduser("~/mnt")
 
 SYSTEM_PROMPT = f"""
 <role>