bug(cli): sandbox help references unpublished Python image

[cwing-nvidia]

User Story

As an OpenShell user, I want the CLI's sandbox examples to reference a published community image so that I can successfully follow the built-in help.

Problem Statement

openshell sandbox --help recommends openshell sandbox create --from python, but OpenShell Community does not publish a python sandbox. The CLI consequently treats python as a local directory and fails because it contains no Dockerfile.

Impact / Why This Matters

Users following the built-in example cannot create a sandbox and may misdiagnose the failure as a gateway or container-runtime problem. The current workaround is to discover a valid image name independently or provide the full community base image reference, which defeats the purpose of the built-in quickstart example.

Acceptance Criteria

• The sandbox help example references a currently published OpenShell Community sandbox.
• The example can resolve without requiring a local Dockerfile named after the example.
• Relevant CLI help tests are updated or added if applicable.

Reproduction Steps

1. Install OpenShell 0.0.111.
2. Run openshell sandbox --help and observe the openshell sandbox create --from python example.
3. Run openshell sandbox create --from python.
4. Observe No Dockerfile found in directory: python.

Environment

• OpenShell: 0.0.111
• OS: macOS 26.5.2 on Apple Silicon
• Runtime, deployment, or integration: Homebrew gateway with Docker through Colima

Logs

Error:   × No Dockerfile found in directory: python