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/cli/generate.go

@@ -175,12 +175,6 @@ func (cli *Instance) GenerateSecret(
 		return fmt.Errorf("failed to generate random secret: %w", err)
 	}
 
-	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
-	if err != nil {
-		return err
-	}
-	defer release()
-
 	// Store the secret in the vault
 	vlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
 	if err != nil {
@@ -193,7 +187,7 @@ func (cli *Instance) GenerateSecret(
 	secretBuffer := memguard.NewBufferFromBytes([]byte(secretValue))
 	defer secretBuffer.Destroy()
 
-	err = vlt.AddSecret(secretName, secretBuffer, force)
+	err = cli.storeSecret(vlt, secretName, secretBuffer, force)
 	if err != nil {
 		return err
 	}