Refuse to create a vault that already exists (closes #74)

vault.CreateVault now checks for the vault before writing anything and
fails with "vault NAME already exists" (vault.ErrVaultExists). secret
init and secret vault create call it while holding the state directory
lock, so two creates at once cannot both pass the check. Before, either
command over an existing vault replaced its metadata, passphrase
unlocker and longterm.age, so none of its secrets could be decrypted.

The lock tests set up the vault "work" instead of "default", which init
now refuses to create again.

Model: opus-5-5

internal/cli/lock_test.go

@@ -274,11 +274,12 @@ func stateDirModTimes(t *testing.T, fs afero.Fs) map[string]int64 {
 }
 
 // setupEveryCommand makes what each command in
-// TestChangingCommandsWaitForLock needs: the current vault "default" with
-// two versions of "test/secret", the vault "other" without a long-term key,
-// for vault import, and the file testInput. If withUnlocker is set, it also
-// gives "default" a passphrase unlocker, which is slow. It returns the older
-// version and the unlocker's ID.
+// TestChangingCommandsWaitForLock needs: the current vault "work" with two
+// versions of "test/secret", the vault "other" without a long-term key, for
+// vault import, and the file testInput. There is no vault "default", which
+// init creates. If withUnlocker is set, it also gives "work" a passphrase
+// unlocker, which is slow. It returns the older version and the unlocker's
+// ID.
 func setupEveryCommand(
 	t *testing.T, fs afero.Fs, withUnlocker bool,
 ) (string, string) {
@@ -291,7 +292,7 @@ func setupEveryCommand(
 	require.NoError(t, err)
 	require.NoError(t, fs.Remove(filepath.Join(otherDir, "pub.age")))
 
-	vlt, err := vault.CreateVault(fs, testStateDir, "default")
+	vlt, err := vault.CreateVault(fs, testStateDir, "work")
 	require.NoError(t, err)
 
 	addTestSecret(t, vlt, []byte("older"), false)