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/cli/version_test.go

@@ -276,8 +276,8 @@ func TestGetSecretWithVersion(t *testing.T) {
 	var buf bytes.Buffer
 	cmd.SetOut(&buf)
 
-	// Test getting current version (empty version string)
-	err = cli.GetSecretWithVersion(cmd, "test/secret", "")
+	// Test getting the current version
+	err = cli.GetSecret(cmd, "test/secret")
 	require.NoError(t, err)
 	assert.Equal(t, "version-2", buf.String())