refactor: return errors from NewCLIInstance instead of panicking

Change NewCLIInstance() and NewCLIInstanceWithFs() to return
(*Instance, error) instead of panicking on DetermineStateDir failure.

Callers in RunE contexts propagate the error. Callers in command
construction (for shell completion) use log.Fatalf. Test callers
use t.Fatalf.

Addresses review feedback on PR #18.

internal/cli/generate.go

@@ -38,7 +38,10 @@ func newGenerateMnemonicCmd() *cobra.Command {
 			`mnemonic phrase that can be used with 'secret init' ` +
 			`or 'secret import'.`,
 		RunE: func(cmd *cobra.Command, _ []string) error {
-			cli := NewCLIInstance()
+			cli, err := NewCLIInstance()
+			if err != nil {
+				return fmt.Errorf("failed to initialize CLI: %w", err)
+			}
 
 			return cli.GenerateMnemonic(cmd)
 		},
@@ -56,7 +59,10 @@ func newGenerateSecretCmd() *cobra.Command {
 			secretType, _ := cmd.Flags().GetString("type")
 			force, _ := cmd.Flags().GetBool("force")
 
-			cli := NewCLIInstance()
+			cli, err := NewCLIInstance()
+			if err != nil {
+				return fmt.Errorf("failed to initialize CLI: %w", err)
+			}
 
 			return cli.GenerateSecret(cmd, args[0], length, secretType, force)
 		},