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, and a forced add fails if another
command created the secret meanwhile, rather than ask under the lock.
secret.CreatePGPUnlocker now takes the vault, already unlocked.

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

Model: opus-5-5

internal/vault/management.go

@@ -235,9 +235,9 @@ func processMnemonicForVault(
 // vault: one wrapping ErrInvalidVaultName when it is not a valid vault name,
 // and one wrapping ErrVaultExists when that vault already exists. Creating an
 // existing vault again would replace its keys, and its secrets could no
-// longer be decrypted. CreateVault checks this before writing anything; the
-// commands that create a vault also check it before they ask for the
-// mnemonic or passphrase, holding the state directory lock.
+// longer be decrypted. The commands that create a vault check it before they
+// ask for the mnemonic or passphrase, and CreateVault checks it again, under
+// the state directory lock, before writing anything.
 func CheckNewVaultName(fs afero.Fs, stateDir string, name string) error {
 	err := ValidateVaultName(name)
 	if err != nil {