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/tests/test_title_generation.py

@@ -0,0 +1,90 @@
+"""Tests for automatic thread title generation."""
+
+import pytest
+
+from src.agents.middlewares.title_middleware import TitleMiddleware
+from src.config.title_config import TitleConfig, get_title_config, set_title_config
+
+
+class TestTitleConfig:
+    """Tests for TitleConfig."""
+
+    def test_default_config(self):
+        """Test default configuration values."""
+        config = TitleConfig()
+        assert config.enabled is True
+        assert config.max_words == 6
+        assert config.max_chars == 60
+        assert config.model_name is None
+
+    def test_custom_config(self):
+        """Test custom configuration."""
+        config = TitleConfig(
+            enabled=False,
+            max_words=10,
+            max_chars=100,
+            model_name="gpt-4",
+        )
+        assert config.enabled is False
+        assert config.max_words == 10
+        assert config.max_chars == 100
+        assert config.model_name == "gpt-4"
+
+    def test_config_validation(self):
+        """Test configuration validation."""
+        # max_words should be between 1 and 20
+        with pytest.raises(ValueError):
+            TitleConfig(max_words=0)
+        with pytest.raises(ValueError):
+            TitleConfig(max_words=21)
+
+        # max_chars should be between 10 and 200
+        with pytest.raises(ValueError):
+            TitleConfig(max_chars=5)
+        with pytest.raises(ValueError):
+            TitleConfig(max_chars=201)
+
+    def test_get_set_config(self):
+        """Test global config getter and setter."""
+        original_config = get_title_config()
+
+        # Set new config
+        new_config = TitleConfig(enabled=False, max_words=10)
+        set_title_config(new_config)
+
+        # Verify it was set
+        assert get_title_config().enabled is False
+        assert get_title_config().max_words == 10
+
+        # Restore original config
+        set_title_config(original_config)
+
+
+class TestTitleMiddleware:
+    """Tests for TitleMiddleware."""
+
+    def test_middleware_initialization(self):
+        """Test middleware can be initialized."""
+        middleware = TitleMiddleware()
+        assert middleware is not None
+        assert middleware.state_schema is not None
+
+    # TODO: Add integration tests with mock Runtime
+    # def test_should_generate_title(self):
+    #     """Test title generation trigger logic."""
+    #     pass
+
+    # def test_generate_title(self):
+    #     """Test title generation."""
+    #     pass
+
+    # def test_after_agent_hook(self):
+    #     """Test after_agent hook."""
+    #     pass
+
+
+# TODO: Add integration tests
+# - Test with real LangGraph runtime
+# - Test title persistence with checkpointer
+# - Test fallback behavior when LLM fails
+# - Test concurrent title generation