Ask for the mnemonic and passphrase before taking the state directory lock (closes #152)

init, vault create, unlocker add, and add, import and generate secret
with --force asked for the mnemonic or passphrase, or unlocked the
vault, while holding the state directory lock, so every other command
that changes anything waited at the prompt. They now ask and unlock
first and lock after. What they checked before asking is checked again
under the lock: unlocker add fails if the vault was removed meanwhile
and refuses a GPG key added meanwhile. A forced add of a secret that
another command created meanwhile lets go of the lock, gets the key,
and takes the lock again. secret.CreatePGPUnlocker now takes the vault,
already unlocked; SetMnemonic and SetUnlockPassphrase, used only by it,
are gone.

Not done: secret move between vaults still unlocks both under the lock;
the issue does not name it.

Model: opus-5-5

internal/vault/vault_error_test.go

@@ -128,7 +128,7 @@ func TestAddSecretForceWithWrongPassphraseChangesNothing(t *testing.T) {
 	t.Cleanup(wrong.Destroy)
 
 	vlt := vault.NewVault(fs, testStateDir, testVaultName)
-	vlt.SetUnlockPassphrase(wrong)
+	vlt.UnlockPassphrase = wrong
 
 	secretDir := filepath.Join(testStateDir, "vaults.d", testVaultName,
 		"secrets.d", testSecretName)