Design checkpoint, pause, and resume support

Motivation

Checkpoint, pause, and resume support would allow long-running or stateful OpenShell sessions to be suspended and restored more ergonomically. This is a larger feature and likely needs gateway/proto design before implementation.

Proposed design scope

Use this issue to agree on the design before implementation. The design should cover:

• What state is captured and what is explicitly out of scope.
• The Python SDK API for checkpoint, pause, and resume.
• Gateway/proto changes, if required.
• Security properties for serialized state.
• Failure modes and compatibility across OpenShell versions.

Acceptance criteria

• Produce a design proposal for checkpoint/pause/resume.
• Identify required gateway/proto changes.
• Define the Python SDK API shape.
• Document security considerations and restore failure modes.
• Split follow-up implementation issues if the design is accepted.

Notes

This is intentionally design-first because it likely crosses SDK, gateway, and protocol boundaries.

Start by mapping the Python SDK, gateway, and proto surfaces involved in OpenShell sessions, then review the issue's proposed design scope. A complete design should define captured state, SDK API shape, gateway/proto changes, security properties, restore failures, version compatibility, and follow-up implementation issues.