Result of a credential delete that may have warnings to surface.
A delete rewrites the whole store, so it can hit the same chmod failures as
a write. warnings carries them symmetrically with WriteOutcome so a
caller's "removed" confirmation doesn't paper over a store the delete
failed to lock down to owner-only.
True if a credential was removed, False if none was stored.
chmod-failure warnings from the rewrite. Always empty for a no-op delete
(removed=False), which performs no write.
