`secret mv --force x x` deleted the secret: a move within one vault
removes an existing destination before renaming the source onto it. The
same happened for `work:x work:`, `work:x work` and `work:x ""`, where an
empty destination defaults to the source name.
moveSecretWithinVault now rejects a move whose two names are the same
before touching anything. A move within a named vault works in that
vault directly instead of selecting it, so the current vault never
changes; the vault must be one of the existing vaults.
The test runs each rejected move on a copy of two in-memory vaults and
requires the exact error and an unchanged state directory.
Model: opus-5-5
