Give every new unlocker a directory of its own (closes #71)

A passphrase unlocker added to a vault that had one, and a PGP, keychain
or Secure Enclave unlocker added on the same day as another of its type,
were written into the existing unlocker's directory file by file, so a
crash part-way left a current unlocker whose files did not belong
together.

Unlocker directories, keychain items and Secure Enclave keys are now
named with the time to the nanosecond, and secret.WriteDir refuses a
directory that exists. Adding a passphrase unlocker writes the new one,
points current-unlocker at it, and only then removes the vault's other
passphrase unlockers.

Model: opus-5-5

internal/cli/path_traversal_test.go

@@ -181,7 +181,7 @@ func TestInvalidSecretNameLeavesVaultsUnchanged(t *testing.T) {
 
 	vaultDir := testStateDir + "/vaults.d/default"
 	require.Contains(t, before, vaultDir+"/secrets.d/x/")
-	require.Contains(t, before, vaultDir+"/unlockers.d/passphrase/")
+	require.Contains(t, before, vaultDir+"/current-unlocker")
 	require.Equal(t, "default", before[testStateDir+"/currentvault"])
 
 	cmd := &cobra.Command{}