Read secret environment variables once per command, then unset them (closes #60)

init and vault create put the mnemonic into the process environment for
vault.CreateVault to read back, so every program they ran, gpg included,
inherited it, and SB_SECRET_MNEMONIC and SB_UNLOCK_PASSPHRASE were read
at 13 places and never unset. Each command that may need them now reads
both once, in its RunE, into locked buffers on the CLI Instance, and
unsets them at once. The buffers are passed down: vault.CreateVault
takes the mnemonic, a Vault carries Mnemonic and UnlockPassphrase, and
the PGP, keychain and Secure Enclave unlocker constructors take both.
Nothing below the command reads the environment. README warns against
both variables.

Model: opus-5-5

internal/cli/generate.go

@@ -76,6 +76,9 @@ func newGenerateSecretCmd() *cobra.Command {
 				return fmt.Errorf("failed to initialize CLI: %w", err)
 			}
 
+			destroySecrets := cli.readSecretEnv()
+			defer destroySecrets()
+
 			return cli.GenerateSecret(cmd, args[0], length, secretType, force)
 		},
 	}
@@ -167,6 +170,8 @@ func (cli *Instance) GenerateSecret(
 		return err
 	}
 
+	vlt.Mnemonic, vlt.UnlockPassphrase = cli.Mnemonic, cli.UnlockPassphrase
+
 	// Protect the generated secret immediately
 	secretBuffer := memguard.NewBufferFromBytes([]byte(secretValue))
 	defer secretBuffer.Destroy()