fix(vendor): discard displaced tree after rollback

[codeforester]

Summary

Make successful vendor rollback destructive by contract: after restoring the previous tree, remove the displaced current tree and report that cleanup. If either move fails, preserve both trees and leave the rollback directory for inspection.

Issue

Fixes #352

Validation

• git diff --check
• shfmt -d -ln bash -i 4 -sr scripts/vendor
• bats tests/vendor.bats -f rollback (2/2)
• env -u BASE_HOME BASE_CACHE_DIR=/private/tmp/base-bash-train-352 ./tests/validate.sh (616 BATS tests and all repository contracts passed)

Demo Impact

None.

Docs Impact

Updated docs/vendor-workflow.md with rollback deletion, failure, disk-usage, and recovery semantics.

API Impact

The scripts/vendor rollback behavior is now explicit: successful rollback discards the displaced tree; failed moves retain recoverable state.

CI Impact

Adds injected move-failure coverage to the vendor contract.

Security Notes

Cleanup remains scoped to the private rollback directory created by the command; move failures fail closed without deleting either tree.

Notes

The rollback contract intentionally does not retain a roll-forward copy after success.

Checklist

• Branch name follows <category>/<issue>-<YYYYMMDD>-<slug>.
• Branch category matches the issue label.
• Pull request is scoped to one issue.
• Pull request body explains changes and validation.
• Documentation is updated for behavior changes.
• Impact sections are complete.
• Pull request includes Fixes #352.