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/derivation_index_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"filippo.io/age"
 	"git.eeqj.de/sneak/secret/pkg/agehd"
 	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
@@ -32,6 +33,7 @@ func (v *realVault) GetFilesystem() afero.Fs { return v.fs }
 // Unused by getLongTermPrivateKey — these satisfy VaultInterface.
 func (v *realVault) AddSecret(string, *memguard.LockedBuffer, bool) error { panic("not used") }
 func (v *realVault) GetCurrentUnlocker() (Unlocker, error)                { panic("not used") }
+func (v *realVault) GetOrDeriveLongTermKey() (*age.X25519Identity, error) { panic("not used") }
 func (v *realVault) CreatePassphraseUnlocker(*memguard.LockedBuffer) (*PassphraseUnlocker, error) {
 	panic("not used")
 }