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 ""`, 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

TODO.md

@@ -25,6 +25,15 @@ 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. Every vault name given with `vault:` must be one
+  of the existing vaults by exact name, so `work:x work/:x` is rejected
+  instead of being taken for a move between two vaults. A move within a
+  named vault no longer makes that vault the current one, whether it
+  succeeds or fails.
 - 2026-10-03: Commands that change the state directory hold one lock
   (`flock` on `lock` in the state directory; a mutex on the in-memory
   test filesystem), so concurrent commands no longer lose versions or