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. Under the lock, unlocker add fails if the vault
was removed, or created again with another key, and refuses a GPG key
added meanwhile; a forced add of a secret created meanwhile lets go of
the lock, gets the key and locks again. secret.CreatePGPUnlocker takes
the unlocked vault; SetMnemonic and SetUnlockPassphrase are gone.

Not done: secret move between vaults still unlocks both under the lock
(#189).

Model: opus-5-5

internal/vault/secrets_version_test.go

@@ -336,7 +336,7 @@ func TestVaultVersionTimestampsAfterUnlock(t *testing.T) {
 
 	// A new instance is locked; AddSecret unlocks it with the passphrase
 	vault := NewVault(fs, testStateDir, "test")
-	vault.SetUnlockPassphrase(passphrase)
+	vault.UnlockPassphrase = passphrase
 
 	addTestSecretToVault(t, vault, testSecretPath, []byte("version-2"), true)
 	require.False(t, fs.readAt.IsZero(), "AddSecret did not unlock the vault")