feat: enhance README and improve environment configuration

- Added a new section in the README for Inference Providers, detailing setup instructions for Nous Portal, OpenRouter, and Custom Endpoints, improving user guidance for LLM connections.
- Updated messaging platform setup instructions to include Slack and WhatsApp, providing clearer steps for configuration.
- Introduced a new environment variable, TERMINAL_SANDBOX_DIR, to allow users to customize the sandbox storage location for Docker and Singularity environments.
- Refactored the Docker and Singularity environment classes to utilize the new sandbox directory for persistent workspaces, enhancing organization and usability.
- Improved handling of working directories across various environments, ensuring compatibility and clarity in execution paths.

tools/environments/modal.py

@@ -50,7 +50,7 @@ class ModalEnvironment(BaseEnvironment):
     def __init__(
         self,
         image: str,
-        cwd: str = "/root",
+        cwd: str = "~",
         timeout: int = 60,
         modal_sandbox_kwargs: Optional[Dict[str, Any]] = None,
         persistent_filesystem: bool = True,