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

README.md

@@ -197,6 +197,9 @@ Creates a new unlocker of the specified type:
 **Options:**
 - `--keyid <id>`: GPG key ID (optional for PGP type, uses default key if not specified)
 
+A vault has one passphrase unlocker: adding one replaces the one the vault
+has, which is removed only once the new one is the current unlocker.
+
 #### `secret unlocker remove <unlocker-id> [--force]` / `secret unlocker rm` ⚠️ 🛑
 
 **DANGER**: Permanently removes an unlocker. Like Unix `rm`, this command
@@ -243,8 +246,8 @@ Decrypts data using an Age key stored as a secret.
 ├── vaults.d/
 │   ├── default/
 │   │   ├── unlockers.d/
-│   │   │   ├── passphrase/              # Passphrase unlocker
-│   │   │   └── pgp/                     # PGP unlocker
+│   │   │   ├── passphrase-<time>/       # Passphrase unlocker
+│   │   │   └── <host>-pgp-<time>/       # PGP unlocker
 │   │   ├── secrets.d/
 │   │   │   ├── api%key/                 # Secret: api/key
 │   │   │   │   ├── versions/
@@ -260,7 +263,7 @@ Decrypts data using an Age key stored as a secret.
 │   │   │       └── current -> versions/20231215.001
 │   │   ├── vault-metadata.json          # Vault metadata
 │   │   ├── pub.age                      # Long-term public key
-│   │   └── current-unlocker -> ../unlockers.d/passphrase
+│   │   └── current-unlocker             # Current unlocker's directory name
 │   └── work/
 │       ├── unlockers.d/
 │       ├── secrets.d/