secret list gives each secret's own creation and last-update times (closes #142)

secret list and secret list --json gave the time of the listing as every
secret's creation and last-update time, because Secret.LoadMetadata set
both to the current time. It now reads file modification times, so
listing still needs no passphrase: the last update is when the secret's
current file was written, and the creation is when its oldest version's
value.age was written.

GetSecretObject no longer loads the times, so secret get reads no more
files than before and keeps its error for a missing current file;
secret list calls LoadMetadata itself.

Judgement call: the creation time comes from value.age, not from the
oldest version's directory, whose time changes when the next add
rewrites that version's metadata.age.

Model: opus-5-5

internal/vault/secrets.go

@@ -427,7 +427,8 @@ func (v *Vault) UnlockVault() (*age.X25519Identity, error) {
 	return longTermIdentity, nil
 }
 
-// GetSecretObject retrieves a Secret object with metadata loaded from this vault
+// GetSecretObject retrieves a Secret object from this vault. Its times are
+// zero until its LoadMetadata reads them.
 func (v *Vault) GetSecretObject(name string) (*secret.Secret, error) {
 	err := ValidateSecretName(name)
 	if err != nil {
@@ -454,16 +455,7 @@ func (v *Vault) GetSecretObject(name string) (*secret.Secret, error) {
 		return nil, fmt.Errorf("secret %s %w", name, ErrSecretNotFound)
 	}
 
-	// Create a Secret object
-	secretObj := secret.NewSecret(v, name)
-
-	// Load the metadata from disk
-	err = secretObj.LoadMetadata()
-	if err != nil {
-		return nil, err
-	}
-
-	return secretObj, nil
+	return secret.NewSecret(v, name), nil
 }
 
 // CopySecretVersion copies a single version from source into destSecretDir