[Docs]: Clarify copied-root teardown expectations after fresh-operator validation

[zhyongrui]

Summary
copied-root teardown expectations after fresh-operator validation

Problem to solve
The current operator documentation at docs/openclawcode/mvp-runbook.md does not explain this point clearly enough for repeated live validation or day-two operations.

Proposed solution
Update docs/openclawcode/mvp-runbook.md with a short, concrete operator note that clarifies the expected behavior and the intended workflow.

• keep the change docs-only
• avoid broad rewrites outside the named document

Acceptance criteria

• docs/openclawcode/mvp-runbook.md documents the behavior clearly enough that a future operator does not need to infer it from dev logs
• no workflow or runtime code changes are required

Severity
Low.