feat(gate): buildSchemaLinkingRequest descriptor builder

harness/.pi/extensions/gate/builders.js

@@ -156,6 +156,24 @@ function buildFreetextRequest({ id, phase, title }) {
   };
 }
 
+// Build a `schema-linking` ui_request: table rows (promote/exclude) each carrying
+// their full catalog columns; the frontend renders rows + a per-table columns
+// modal (suggested pre-selected). `tables` is validated shallowly here.
+function buildSchemaLinkingRequest({ id, phase, title, tables }) {
+  requireString(title, "title", "schema-linking");
+  const tabs = requireArray(tables, "tables", "schema-linking");
+  return {
+    type: "ui_request",
+    id,
+    phase,
+    schema_version: SCHEMA_VERSION,
+    widget: "schema-linking",
+    title,
+    tables: [...tabs],
+    reserved: RESERVED,
+  };
+}
+
 // Attach a child-widget spec to an option (linkage, §4.2). Returns the option.
 function withChildLinkage(option, widgetSpec) {
   option.opens = widgetSpec;
@@ -168,6 +186,7 @@ module.exports = {
   buildArtifactGate,
   buildInfoRequest,
   buildFreetextRequest,
+  buildSchemaLinkingRequest,
   withChildLinkage,
   SCHEMA_VERSION,
   RESERVED,