Make another unlocker current when the current one is removed (closes #137)

Removing the vault's current unlocker left current-unlocker naming it,
so every command that opened the vault failed although another
unlocker remained, and the error pointed at the mnemonic.
RemoveUnlocker now first makes current the first other unlocker that
`secret unlocker select` accepts, in the order `secret unlocker list`
shows, and returns its ID, which `secret unlocker remove` prints; with
none, current-unlocker is removed too. When current-unlocker names an
unlocker that does not exist and the vault has another that select
accepts, the error wraps ErrUnlockerNotFound and says to run
`secret unlocker select`, without the mnemonic advice.

Unverified: no test covers that advice for a vault other than the
current one, which gets `secret vault select` first.

Model: opus-5-5

TODO.md

@@ -18,6 +18,19 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-14: Removing the current unlocker no longer leaves the vault unable to
+  open through the unlockers it still has
+  (https://git.eeqj.de/sneak/secret/issues/137). `current-unlocker` went on
+  naming the removed unlocker, so every command that opened the vault failed,
+  with advice to use the mnemonic. `secret unlocker remove` of the current
+  unlocker now makes the first of the vault's other unlockers that
+  `secret unlocker select` accepts, in the order `secret unlocker list` shows,
+  current before removing it, and says so; when there is none, it also removes
+  `current-unlocker`. `vault.RemoveUnlocker` returns the ID of the unlocker it
+  made current. When `current-unlocker` names an unlocker that does not exist
+  and the vault has another that `secret unlocker select` accepts, the error
+  wraps `vault.ErrUnlockerNotFound` and says to run `secret unlocker select`,
+  without the advice to use the mnemonic.
 - 2026-10-14: A `secret add --force` that cannot unlock the vault, given a wrong
   passphrase or mnemonic, leaves the secret as it was
   (https://git.eeqj.de/sneak/secret/issues/136). The new version used to be