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/pyproject.toml

@@ -5,6 +5,8 @@ description = "Add your description here"
 readme = "README.md"
 requires-python = ">=3.12"
 dependencies = [
+    "agent-sandbox>=0.0.19",
+    "dotenv>=0.9.9",
     "langchain>=1.2.3",
     "langchain-deepseek>=1.0.1",
     "langchain-openai>=1.1.7",
@@ -18,5 +20,6 @@ dependencies = [
 
 [dependency-groups]
 dev = [
+    "pytest>=8.0.0",
     "ruff>=0.14.11",
 ]