bug: do not orphan the displaced vendor tree after rollback

[codeforester]

Related roadmap: #214

Problem

A successful scripts/vendor rollback DESTINATION moves the current tree into a new DESTINATION.failed.XXXXXX/current directory, restores DESTINATION.previous, and returns success without deleting or reporting the displaced tree.

Every rollback can therefore leave a hidden full framework copy beside the destination. The retained tree has no documented roll-forward command or cleanup lifecycle.

Evidence

Reviewed on main at 63753047312cd5c189600ed71597faceae0f5d8e.

A create → update → rollback sequence leaves a directory such as:

vendor.failed.nK4Dzm/current

The success message reports only the restored destination. The successful path at scripts/vendor:146-155 has no cleanup or ownership transfer for the temporary directory, and the vendor workflow documentation mentions only .previous.

Acceptance criteria

• Define whether the displaced current tree is deleted or retained for an explicit roll-forward/recovery contract.
• Do not leave an unreported orphan after successful rollback.
• If retained, use a stable documented path, report it, verify it, and define replacement/cleanup behavior.
• Preserve failure-safe restoration if either move fails.
• Add filesystem-state assertions after successful rollback and after injected move failures.
• Document disk-usage and recovery semantics.