Make secret unlocker add pgp work on Linux (closes #88)

CreatePGPUnlocker got the vault's long-term key from the keychain
unlocker's helper, which on every platform but macOS is a stub that
always fails. It now calls the vault's GetOrDeriveLongTermKey, as adding
a passphrase unlocker does: from the mnemonic, checked against the
vault, or else from the current unlocker. That method joins
VaultInterface. The test GPG key gains an encryption subkey, and a new
test adds a PGP unlocker with the long-term key from the mnemonic and
from a passphrase unlocker, then reads a secret through it.

Model: opus-5-5

internal/secret/keychainunlocker_stub.go

@@ -6,7 +6,6 @@ import (
 	"errors"
 
 	"filippo.io/age"
-	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
 )
 
@@ -79,10 +78,3 @@ func (k *KeychainUnlocker) Remove() error {
 func CreateKeychainUnlocker(_ afero.Fs, _ string) (*KeychainUnlocker, error) {
 	return nil, errKeychainNotSupported
 }
-
-// getLongTermPrivateKey returns an error on non-Darwin platforms
-func getLongTermPrivateKey(
-	_ afero.Fs, _ VaultInterface,
-) (*memguard.LockedBuffer, error) {
-	return nil, errKeychainNotSupported
-}