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/secret/atomic.go

@@ -3,6 +3,7 @@ package secret
 import (
 	"errors"
 	"fmt"
+	"os"
 	"path/filepath"
 
 	"github.com/spf13/afero"
@@ -62,13 +63,12 @@ func TempDirFor(fs afero.Fs, target string) (string, error) {
 	return dir, nil
 }
 
-// WriteDir calls write to write the files of the directory dir. When dir does
-// not exist yet, write writes them into a temporary directory from TempDirFor,
-// which is then renamed to dir, so that neither a failure nor a crash leaves
-// dir half-written; on a failure the temporary directory is removed, and a
-// failure to remove it is returned along with the first. A directory cannot be
-// renamed over one that has files in it, so when dir already exists, write
-// writes into it in place; dir is then never removed.
+// WriteDir calls write to write the files of the new directory dir into a
+// temporary directory from TempDirFor, which is then renamed to dir, so that
+// neither a failure nor a crash leaves dir half-written; on a failure the
+// temporary directory is removed, and a failure to remove it is returned
+// along with the first. A directory cannot be replaced in one rename, so if
+// dir already exists, WriteDir fails without calling write.
 func WriteDir(fs afero.Fs, dir string, write func(dir string) error) error {
 	exists, err := afero.Exists(fs, dir)
 	if err != nil {
@@ -76,7 +76,7 @@ func WriteDir(fs afero.Fs, dir string, write func(dir string) error) error {
 	}
 
 	if exists {
-		return write(dir)
+		return fmt.Errorf("failed to create %s: %w", dir, os.ErrExist)
 	}
 
 	// Create the directory the finished one is renamed into