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/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>