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/vault.go

@@ -267,6 +267,12 @@ func (cli *Instance) CreateVault(cmd *cobra.Command, name string) error {
 	}
 	defer release()
 
+	// Refuse an invalid name or an existing vault before asking for anything
+	err = vault.CheckNewVaultName(cli.fs, cli.stateDir, name)
+	if err != nil {
+		return err
+	}
+
 	mnemonic, cleanupMnemonic, err := cli.promptMnemonic()
 	if err != nil {
 		return err