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 makes the first other unlocker, in the order
`secret unlocker list` shows, current before the removal and returns
its ID, which `secret unlocker remove` prints; removing the last
unlocker also removes current-unlocker. When current-unlocker names an
unlocker that does not exist and the vault has others, 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

internal/cli/unlockers.go

@@ -222,8 +222,10 @@ func newUnlockerRemoveCmd() *cobra.Command {
 		Long: `Remove an unlocker from the current vault. Asks for ` +
 			`confirmation first, saying whether it is the vault's last ` +
 			`unlocker; when stdin is not a terminal, fails unless --force ` +
-			`is given. Warning: Without unlockers and without your ` +
-			`mnemonic, vault data will be permanently inaccessible.`,
+			`is given. Removing the current unlocker makes another of the ` +
+			`vault's unlockers current, if it has one. Warning: Without ` +
+			`unlockers and without your mnemonic, vault data will be ` +
+			`permanently inaccessible.`,
 		Args:              cobra.ExactArgs(1),
 		ValidArgsFunction: getUnlockerIDsCompletionFunc(cli.fs, cli.stateDir),
 		RunE: func(cmd *cobra.Command, args []string) error {
@@ -612,13 +614,17 @@ func (cli *Instance) removeUnlocker(
 			"have your mnemonic phrase to access this vault again!")
 	}
 
-	err := found.vlt.RemoveUnlocker(unlockerID)
+	newCurrentID, err := found.vlt.RemoveUnlocker(unlockerID)
 	if err != nil {
 		return err
 	}
 
 	cmd.Printf("Removed unlocker '%s'\n", unlockerID)
 
+	if newCurrentID != "" {
+		cmd.Printf("Switched current unlocker to '%s'\n", newCurrentID)
+	}
+
 	return nil
 }