fix: reserve TRIGGER_PREVIEW_BRANCH from external sync

Author: 0ski

apps/webapp/app/v3/environmentVariableRules.server.ts

@@ -10,15 +10,10 @@ const blacklistedVariables: VariableRule[] = [
   { type: "exact", key: "TRIGGER_API_URL" },
 ];
 
-// Deploy-managed keys that are not blacklisted (so users may set them manually),
-// but should never be pulled in from an external integration like Vercel because
-// the platform owns their value.
-const additionalExternalSyncReservedKeys = ["TRIGGER_VERSION"];
-
-/**
- * Returns true if a key would be stripped by `removeBlacklistedVariables`, i.e. the
- * environment variable repository will refuse to store it.
- */
+// Platform-managed keys that aren't blacklisted but should never be synced from an
+// external integration. Keep in sync with the cloud repo's EXCLUDED_ENV_VAR_KEYS.
+const additionalExternalSyncReservedKeys = ["TRIGGER_VERSION", "TRIGGER_PREVIEW_BRANCH"];
+
 export function isBlacklistedVariable(key: string): boolean {
   const whitelisted = blacklistedVariables.find((bv) => bv.type === "whitelist" && bv.key === key);
   if (whitelisted) {