Cover both workspace delete branches in manager_tests
The existing cabinet delete regression test guards only one of three
paths delete() can take. The two workspace paths — owner (atomic apply)
and non-owner (directoryUpdate proposal) — had no coverage, so a
refactor that flipped the Applied/Proposed branch would have passed
cargo test silently.
Add tests for both. The owner test mirrors the cabinet shape
(getRecord + applyWrites with [delete, update]) and asserts
MutationOutcome::Applied. The non-owner test skips the getRecord
(proposal path doesn't touch the parent directory) and asserts the
single applyWrites batch carries [delete, createProposal], with the
proposal round-tripping to a RemoveEntry variant carrying the
correct keyring, directory, and entry URIs.