Read secret environment variables once per command, then unset them (closes #60)

init and vault create put the mnemonic into the process environment for
vault.CreateVault to read back, so every program they ran, gpg included,
inherited it, and SB_SECRET_MNEMONIC and SB_UNLOCK_PASSPHRASE were read
at 13 places and never unset. Each command that may need them now reads
both once, in its RunE, into locked buffers on the CLI Instance, and
unsets them at once. The buffers are passed down: vault.CreateVault
takes the mnemonic, a Vault carries Mnemonic and UnlockPassphrase, and
the PGP, keychain and Secure Enclave unlocker constructors take both;
CreatePGPUnlocker sets them on the vault it loads through SetMnemonic
and SetUnlockPassphrase, new in VaultInterface. README warns against
both variables.

Model: opus-5-5

internal/secret/pgpunlocker.go

@@ -254,9 +254,12 @@ func pgpUnlockerDir(
 // fingerprint as ResolveGPGKeyFingerprint returns it, in the metadata.
 // Everything that can fail short of writing a file is done before anything
 // is written, and the files are written through WriteDir, so a failure
-// leaves no partial unlocker.
+// leaves no partial unlocker. The long-term key comes from mnemonic when it
+// is not nil, else from the current unlocker, which is given passphrase when
+// it is a passphrase unlocker.
 func CreatePGPUnlocker(
 	fs afero.Fs, stateDir, gpgKeyID, fingerprint string,
+	mnemonic, passphrase *memguard.LockedBuffer,
 ) (*PGPUnlocker, error) {
 	err := checkGPGAvailable()
 	if err != nil {
@@ -268,6 +271,10 @@ func CreatePGPUnlocker(
 		return nil, err
 	}
 
+	// The vault's GetOrDeriveLongTermKey, in step 2, uses both
+	vault.SetMnemonic(mnemonic)
+	vault.SetUnlockPassphrase(passphrase)
+
 	// Step 1: Generate a new age keypair for the PGP unlocker
 	ageIdentity, err := age.GenerateX25519Identity()
 	if err != nil {