`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 ""`, and for
`work:x work/:x`, which named one vault two ways and so was taken for a
move between vaults.
A move whose two names are the same is now rejected before anything
changes. Every vault named with `vault:` must be one of the existing
vaults by exact name, checked before choosing between the two kinds of
move. A move within a named vault no longer makes it the current vault.
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
