fix: resolve errcheck, gosec, and mnd linter errors

- Fixed unhandled errors in init.go (os.Setenv/Unsetenv)
- Fixed unhandled errors in test_helpers.go (os.Setenv/Unsetenv)
- Replaced magic numbers with named constants:
  - defaultSecretLength = 16
  - mnemonicEntropyBits = 128
  - tabWriterPadding = 2

internal/cli/init.go

@@ -80,12 +80,12 @@ func (cli *Instance) Init(cmd *cobra.Command) error {
 
 	// Set mnemonic in environment for CreateVault to use
 	originalMnemonic := os.Getenv(secret.EnvMnemonic)
-	os.Setenv(secret.EnvMnemonic, mnemonicStr)
+	_ = os.Setenv(secret.EnvMnemonic, mnemonicStr)
 	defer func() {
 		if originalMnemonic != "" {
-			os.Setenv(secret.EnvMnemonic, originalMnemonic)
+			_ = os.Setenv(secret.EnvMnemonic, originalMnemonic)
 		} else {
-			os.Unsetenv(secret.EnvMnemonic)
+			_ = os.Unsetenv(secret.EnvMnemonic)
 		}
 	}()