Add high-level execute(code) helper for Python execution

Motivation

OpenShell already exposes lower-level execution paths, but common Python code execution should be available through a small, ergonomic helper. A high-level execute(code) API would make the Python SDK easier to use for agent and notebook-style workflows without changing the underlying protocol.

Proposed change

Add a Python SDK method similar to:

result = sandbox.execute("print('hello from OpenShell')")

The helper should wrap the existing exec_python / exec path where appropriate, preserve the lower-level APIs, and return the same execution result information callers can inspect today.

Acceptance criteria

• Add a documented high-level execute(code: str, ...) helper to the Python SDK.
• Preserve existing lower-level execution APIs without breaking changes.
• Include unit tests for success, failure, timeout/error propagation, and result shape.
• Add a README or docs example showing simple Python execution.

Notes

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

Start in the Python SDK at the existing exec_python and exec paths to understand their result shape, error handling, and timeout behavior. Add the documented execute(code: str, ...) helper while preserving those lower-level APIs, then run or add unit tests for success, failure, timeout/error propagation, and result shape, and update the README or docs with a simple example.