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/config/title_config.py

@@ -0,0 +1,53 @@
+"""Configuration for automatic thread title generation."""
+
+from pydantic import BaseModel, Field
+
+
+class TitleConfig(BaseModel):
+    """Configuration for automatic thread title generation."""
+
+    enabled: bool = Field(
+        default=True,
+        description="Whether to enable automatic title generation",
+    )
+    max_words: int = Field(
+        default=6,
+        ge=1,
+        le=20,
+        description="Maximum number of words in the generated title",
+    )
+    max_chars: int = Field(
+        default=60,
+        ge=10,
+        le=200,
+        description="Maximum number of characters in the generated title",
+    )
+    model_name: str | None = Field(
+        default=None,
+        description="Model name to use for title generation (None = use default model)",
+    )
+    prompt_template: str = Field(
+        default=("Generate a concise title (max {max_words} words) for this conversation.\nUser: {user_msg}\nAssistant: {assistant_msg}\n\nReturn ONLY the title, no quotes, no explanation."),
+        description="Prompt template for title generation",
+    )
+
+
+# Global configuration instance
+_title_config: TitleConfig = TitleConfig()
+
+
+def get_title_config() -> TitleConfig:
+    """Get the current title configuration."""
+    return _title_config
+
+
+def set_title_config(config: TitleConfig) -> None:
+    """Set the title configuration."""
+    global _title_config
+    _title_config = config
+
+
+def load_title_config_from_dict(config_dict: dict) -> None:
+    """Load title configuration from a dictionary."""
+    global _title_config
+    _title_config = TitleConfig(**config_dict)