Refuse an existing vault before asking for the mnemonic (closes #151)

`secret init` and `secret vault create` asked for the mnemonic and the
unlocker passphrase, and `secret init` printed "Initialized secrets
manager at", before refusing a vault that already exists. The check
`vault.CreateVault` made before writing is now `vault.CheckNewVaultName`,
which both commands call while holding the state directory lock, before
the first prompt; `CreateVault` still calls it. `secret init` prints the
"Initialized" line only once the vault is created, so
`TestMnemonicNotReadNamesOnlyMnemonic` no longer expects it.

Judgement call: the same check validates the name, so `secret vault
create` with an invalid name also fails before the prompts.

Model: opus-5-5

internal/cli/create_vault_test.go

@@ -222,12 +222,51 @@ func TestMnemonicNotReadNamesOnlyMnemonic(t *testing.T) {
 	output, err := cmd.CombinedOutput()
 	require.Error(t, err)
 
-	require.Equal(t, "Initialized secrets manager at: "+stateDir+"\n"+
-		"Error: failed to read mnemonic: there is no terminal to ask on. "+
-		"Please set the SB_SECRET_MNEMONIC environment variable or run "+
+	require.Equal(t, "Error: failed to read mnemonic: there is no terminal to "+
+		"ask on. Please set the SB_SECRET_MNEMONIC environment variable or run "+
 		"interactively\n", string(output))
 }
 
+// TestCreateExistingVaultAsksForNothing is a regression test for
+// https://git.eeqj.de/sneak/secret/issues/151: `secret init` and `secret
+// vault create` asked for the mnemonic and the unlocker passphrase before
+// refusing a vault that already exists, and `secret init` printed
+// "Initialized secrets manager at" first. Each runs here as the built binary,
+// in a new session, which has no terminal, and with neither
+// SB_SECRET_MNEMONIC nor SB_UNLOCK_PASSPHRASE set, so asking for either
+// fails. Each must fail saying only that the vault exists.
+func TestCreateExistingVaultAsksForNothing(t *testing.T) {
+	t.Parallel()
+
+	stateDir := t.TempDir()
+	_, err := vault.CreateVault(afero.NewOsFs(), stateDir, "default", nil, nil)
+	require.NoError(t, err)
+
+	// run runs the built binary with args, which must fail, and returns what
+	// it printed
+	run := func(args ...string) string {
+		//nolint:gosec // G204: test executes the freshly built secret binary
+		cmd := exec.CommandContext(t.Context(), secretBinaryPath(t), args...)
+		cmd.Env = []string{
+			secret.EnvStateDir + "=" + stateDir,
+			"PATH=" + os.Getenv("PATH"),
+			"HOME=" + os.Getenv("HOME"),
+		}
+		cmd.SysProcAttr = &syscall.SysProcAttr{Setsid: true}
+
+		output, err := cmd.CombinedOutput()
+		require.Error(t, err, "secret %v", args)
+
+		return string(output)
+	}
+
+	require.Equal(t,
+		"Error: failed to create default vault: vault default already exists\n",
+		run("init"))
+	require.Equal(t, "Error: vault default already exists\n",
+		run("vault", "create", "default"))
+}
+
 // TestStopDuringCreateLeavesWholeVaultOrNone is a regression test for
 // https://git.eeqj.de/sneak/secret/issues/105: `secret init` or `secret vault
 // create` killed after the passphrase prompt but before the unlocker was