Add from_env() configuration helper for OpenShell Python SDK

[dhirajsb]

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.

[dhirajsb]

@NVIDIA/openshell-codeowners, could you please help with triage on this SDK request?

Would an explicit from_env() configuration helper be useful for CI, container, and developer-shell integrations? If so, may I update my existing implementation and submit it as a PR after the environment-variable names and precedence are agreed?

If RFC-0008 (#1764), #2565, or another SDK/configuration effort already owns this behavior, please point me to it so I can avoid introducing a competing public API.