Accept a version argument only if the secret has that version (closes #67)

version rm, version promote and get --version joined the version
argument into a path unchecked, so "", ".", "..", "../../.." removed or
read every version, the secret, the vault or directories above it.

A version is now accepted only if it is one of the versions
ListVersions lists for the secret, compared by name before any path is
built (secret.VersionExists, used by all three). An empty --version is
rejected instead of meaning the current version: GetSecretVersion no
longer treats "" as current, and GetSecret looks the current version
up itself.

Model: opus-5-5

internal/vault/secrets_version_test.go

@@ -202,10 +202,10 @@ func TestVaultGetSecretVersion(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, []byte("version-2"), value)
 
-	// Get current (empty version)
-	value, err = vault.GetSecretVersion(testSecretPath, "")
-	require.NoError(t, err)
-	assert.Equal(t, []byte("version-2"), value)
+	// An empty version is not one of the versions; GetSecret gets the
+	// current one
+	_, err = vault.GetSecretVersion(testSecretPath, "")
+	require.ErrorIs(t, err, ErrVersionNotFound)
 }
 
 //nolint:paralleltest // createTestVaultWithKey uses t.Setenv