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/sandbox/middleware.py

@@ -1,4 +1,4 @@
-from typing import override
+from typing import NotRequired, override
 
 from langchain.agents import AgentState
 from langchain.agents.middleware import AgentMiddleware
@@ -11,7 +11,7 @@ from src.sandbox import get_sandbox_provider
 class SandboxMiddlewareState(AgentState):
     """Compatible with the `ThreadState` schema."""
 
-    sandbox: SandboxState | None = None
+    sandbox: NotRequired[SandboxState | None]
 
 
 class SandboxMiddleware(AgentMiddleware[SandboxMiddlewareState]):