Preserve playground readiness diagnostics

[rmcdaniel]

Closes #75.

What changed

• include the failed command, exit status, stderr, and stdout when a JSON-producing CLI command fails
• retain the strict worker registration proof and preserve the final actionable roster error
• add focused subprocess and timeout regression coverage

Local verification

• Python AST parse
• Pint on PlaygroundContractTest
• focused readiness tests passed
• the complete PlaygroundContractTest reached 14 passing tests; two environment-only cases require the image-baked SDK runtime and a Git worktree inside the test container, both supplied by GitHub Actions

The underlying stable Server compatibility correction is durable-workflow/server#89.