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/crypto.go

@@ -22,7 +22,10 @@ func newEncryptCmd() *cobra.Command {
 			inputFile, _ := cmd.Flags().GetString("input")
 			outputFile, _ := cmd.Flags().GetString("output")
 
-			cli := NewCLIInstance()
+			cli, err := NewCLIInstance()
+			if err != nil {
+				return fmt.Errorf("failed to initialize CLI: %w", err)
+			}
 			cli.cmd = cmd
 
 			return cli.Encrypt(args[0], inputFile, outputFile)
@@ -45,7 +48,10 @@ func newDecryptCmd() *cobra.Command {
 			inputFile, _ := cmd.Flags().GetString("input")
 			outputFile, _ := cmd.Flags().GetString("output")
 
-			cli := NewCLIInstance()
+			cli, err := NewCLIInstance()
+			if err != nil {
+				return fmt.Errorf("failed to initialize CLI: %w", err)
+			}
 			cli.cmd = cmd
 
 			return cli.Decrypt(args[0], inputFile, outputFile)