Stop vault safety checks from reading unreadable state as empty (closes #51)

Adding a PGP unlocker checked unlockers.d for a duplicate and, when the
directory could not be read, reported no duplicate and went on. The
check now returns an error naming the directory and cause, and the add
stops; a duplicate found is still reported as one.

The same flaw guarded removing the last unlocker and removing a vault
(an unreadable secrets directory counted as no secrets) and vault
import (an unreadable pub.age counted as no long-term key). Those now
stop with an error too.

`unlocker list` keeps skipping entries it cannot read; a comment at the
duplicate check says why the two differ.

`vault rm` and `unlocker rm` now take the state directory lock and call
an unexported function that does the work, as `vault import` does, so
the tests can reach their checks.

Model: opus-5-5

internal/cli/vault.go

@@ -400,8 +400,12 @@ func (cli *Instance) vaultImportPreflight(
 	// Check if vault already has a public key
 	pubKeyPath := vaultDir + "/pub.age"
 
-	_, err = cli.fs.Stat(pubKeyPath)
-	if err == nil {
+	exists, err = afero.Exists(cli.fs, pubKeyPath)
+	if err != nil {
+		return "", "", "", fmt.Errorf("failed to check %s: %w", pubKeyPath, err)
+	}
+
+	if exists {
 		return "", "", "", fmt.Errorf("vault '%s' %w",
 			vaultName, errVaultHasLongTermKey)
 	}
@@ -561,17 +565,26 @@ func (cli *Instance) importMnemonic(cmd *cobra.Command, vaultName string) error
 }
 
 // vaultHasSecrets reports whether the vault directory contains any secrets
-func (cli *Instance) vaultHasSecrets(vaultDir string) bool {
+func (cli *Instance) vaultHasSecrets(vaultDir string) (bool, error) {
 	secretsDir := filepath.Join(vaultDir, "secrets.d")
 
-	exists, _ := afero.DirExists(cli.fs, secretsDir)
+	exists, err := afero.DirExists(cli.fs, secretsDir)
+	if err != nil {
+		return false, fmt.Errorf("failed to check secrets directory %s: %w",
+			secretsDir, err)
+	}
+
 	if !exists {
-		return false
+		return false, nil
 	}
 
 	entries, err := afero.ReadDir(cli.fs, secretsDir)
+	if err != nil {
+		return false, fmt.Errorf("failed to read secrets directory %s: %w",
+			secretsDir, err)
+	}
 
-	return err == nil && len(entries) > 0
+	return len(entries) > 0, nil
 }
 
 // switchAwayFromVault selects another vault as current before removal
@@ -600,7 +613,8 @@ func (cli *Instance) switchAwayFromVault(
 	return nil
 }
 
-// RemoveVault removes a vault with safety checks
+// RemoveVault removes a vault, holding the state directory lock while
+// removeVault runs
 func (cli *Instance) RemoveVault(cmd *cobra.Command, name string, force bool) error {
 	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
 	if err != nil {
@@ -608,6 +622,11 @@ func (cli *Instance) RemoveVault(cmd *cobra.Command, name string, force bool) er
 	}
 	defer release()
 
+	return cli.removeVault(cmd, name, force)
+}
+
+// removeVault removes a vault with safety checks
+func (cli *Instance) removeVault(cmd *cobra.Command, name string, force bool) error {
 	// Get list of all vaults
 	vaults, err := vault.ListVaults(cli.fs, cli.stateDir)
 	if err != nil {
@@ -641,7 +660,10 @@ func (cli *Instance) RemoveVault(cmd *cobra.Command, name string, force bool) er
 	}
 
 	// Check if vault has secrets
-	hasSecrets := cli.vaultHasSecrets(vaultDir)
+	hasSecrets, err := cli.vaultHasSecrets(vaultDir)
+	if err != nil {
+		return err
+	}
 
 	// Require --force if vault has secrets
 	if hasSecrets && !force {