Commands fail when currentvault names no vault (closes #156)

An empty currentvault, or one naming a vault that does not exist, made
`secret list` report an empty vault with exit 0 and `secret add` fail on a
missing directory. ResolveVaultSymlink now refuses an empty or invalid
name, or one with no directory under vaults.d, with an error that wraps
ErrVaultNotFound, names the file and says to run
`secret vault select <name>`. Every command that uses the current vault
gets it; `secret vault list` and `secret info` still run.

newSizeTestVault wrote a full path into currentvault, which resolved only
because the last path element was taken as the name; it now relies on
CreateVault selecting the vault.

Model: opus-5-5

internal/vault/management.go

@@ -55,7 +55,10 @@ func ValidateVaultName(name string) error {
 }
 
 // ResolveVaultSymlink reads the currentvault file to get the path to the current vault
-// The file contains just the vault name (e.g., "default")
+// The file contains just the vault name (e.g., "default"). An empty or
+// invalid name, or the name of a vault that does not exist, is an error
+// wrapping ErrVaultNotFound that names the file and says to run
+// 'secret vault select <name>'.
 func ResolveVaultSymlink(fs afero.Fs, currentVaultPath string) (string, error) {
 	secret.Debug("resolveVaultSymlink starting", "path", currentVaultPath)
 
@@ -74,6 +77,21 @@ func ResolveVaultSymlink(fs afero.Fs, currentVaultPath string) (string, error) {
 	stateDir := filepath.Dir(currentVaultPath)
 	absolutePath := filepath.Join(stateDir, "vaults.d", vaultName)
 
+	// An empty name would make vaults.d itself the vault's directory
+	exists := false
+	if isValidVaultName(vaultName) {
+		exists, err = afero.DirExists(fs, absolutePath)
+		if err != nil {
+			return "", fmt.Errorf("failed to check if vault exists: %w", err)
+		}
+	}
+
+	if !exists {
+		return "", fmt.Errorf("vault '%s' named in %s %w; run "+
+			"'secret vault select <name>' to select a vault",
+			vaultName, currentVaultPath, ErrVaultNotFound)
+	}
+
 	secret.Debug("Resolved to absolute path", "absolute_path", absolutePath)
 
 	return absolutePath, nil