Stop secret mv deleting a secret moved onto itself (closes #73)

`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

TODO.md

@@ -25,6 +25,12 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-03: `secret mv` rejects a move whose destination is the
+  source (`mv --force x x`, `mv --force work:x work:`, or an empty
+  destination, which defaults to the source name) before changing
+  anything; before, `--force` removed the destination first and so
+  deleted the secret. A move within a named vault no longer makes that
+  vault the current one, whether it succeeds or fails.
 - 2026-10-03: Every command that builds a path from a secret name
   checks the name first with `vault.ValidateSecretName` and touches
   nothing when it is invalid: `rm`, `mv` (both names, within a vault