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

internal/vault/vault.go

@@ -291,9 +291,11 @@ func (v *Vault) unlockLongTermKey(
 // without a mnemonic records none, and without its metadata the key cannot
 // be derived. It is not added when the passphrase could not be read: the
 // unlocker was not tried, and adding one would need a passphrase read the
-// same way.
+// same way. Nor is it added when the current unlocker does not exist and the
+// vault has others: the error already says to select one of them.
 func (v *Vault) withMnemonicAdvice(err error) error {
-	if errors.Is(err, secret.ErrPassphraseNotRead) {
+	if errors.Is(err, secret.ErrPassphraseNotRead) ||
+		errors.Is(err, ErrUnlockerNotFound) {
 		return err
 	}
 
@@ -304,15 +306,21 @@ func (v *Vault) withMnemonicAdvice(err error) error {
 		return err
 	}
 
-	// 'secret unlocker add' acts on the current vault only.
-	steps := "'secret unlocker add passphrase'"
-
-	current, currentErr := GetCurrentVault(v.fs, v.stateDir)
-	if currentErr != nil || current.Name != v.Name {
-		steps = fmt.Sprintf("'secret vault select %s', then %s", v.Name, steps)
-	}
-
 	return fmt.Errorf("%w; the vault '%s' still opens with its mnemonic: run "+
 		"%s with %s set to the mnemonic to give it a new unlocker",
-		err, v.Name, steps, secret.EnvMnemonic)
+		err, v.Name, v.withVaultSelect("'secret unlocker add passphrase'"),
+		secret.EnvMnemonic)
+}
+
+// withVaultSelect returns command, a quoted 'secret unlocker' command, for
+// advice about this vault. Such a command acts on the current vault only,
+// so when this vault is not the current one, the command that selects it
+// comes first.
+func (v *Vault) withVaultSelect(command string) string {
+	current, err := GetCurrentVault(v.fs, v.stateDir)
+	if err != nil || current.Name != v.Name {
+		return fmt.Sprintf("'secret vault select %s', then %s", v.Name, command)
+	}
+
+	return command
 }