feat(examples): add Jupyter sandbox fleet

[drew]

Summary

Add a Python SDK example that launches a configurable fleet of Jupyter sandboxes, exposes each Jupyter API server as an OpenShell service, and submits code to one member through the Jupyter REST and WebSocket APIs.

Related Issue

Closes #2252

Changes

• Add a generic context-managed Fleet abstraction with partial-failure cleanup.
• Manage Jupyter sandbox lifecycle through the OpenShell Python SDK.
• Add a configurable plain Python demo that launches three fleet members and submits work to one member.
• Add a pinned Jupyter image, restrictive sandbox policy, and usage documentation.
• Document missing Python SDK APIs and retain the CLI only for service expose/delete operations.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

No example test files were added per the current scope. Focused Ruff, formatting, ty, policy/Fleet smoke checks, Markdown lint, SPDX checks, and the Docker image build also passed. A live sandbox run could not be completed because the available local gateways were unhealthy or unready.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; the example README documents the feature)

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