Reject invalid and cross-company execution workspace references with a useful
422 before changing the work product. Hold the validated reference through
the transaction so concurrent deletion cannot turn validation into a 500.
Verified 13 focused tests, full typecheck/build, and green PR CI. Greptile
5/5 with no unresolved comments. The known UI copy-toast flake passed on an
unchanged-commit retry and in a focused local run.
Co-Authored-By: Paperclip <noreply@paperclip.ing>