Add from_env() configuration helper for OpenShell Python SDK

Motivation

The Python SDK should be easy to configure in CI, containers, and developer shells. A from_env() helper would make OpenShell configuration explicit and repeatable while complementing existing XDG / OPENSHELL_GATEWAY discovery.

Proposed change

Add a config constructor or client factory that loads OpenShell settings from environment variables, for example gateway address, auth/material paths, and timeout defaults.

Example shape:

client = openshell.Client.from_env()

or:

config = openshell.Config.from_env()
client = openshell.Client(config)

Acceptance criteria

• Define and document the supported environment variables and precedence rules.
• Add from_env() tests for defaults, explicit overrides, and invalid values.
• Keep existing configuration/discovery behavior backward compatible.
• Include a short docs example for local development and CI usage.

Notes

This should be SDK-only and should not require gateway/proto changes.

Locate the Python SDK's existing XDG and OPENSHELL_GATEWAY discovery and configuration entry points first. Define the supported environment variables and precedence, then add from_env() tests covering defaults, overrides, and invalid values. Done means existing discovery remains compatible and a short local-development and CI example is documented.