feat: Phase 0 workspace, spikes, and golden harness

Stand up the monorepo skeleton and land the first Phase 0 artifacts:
pairing threat model, plugin catalog, builder detect, 20-prompt golden
harness, and synthetic 2GB mirror timing.

e2e/golden/src/builder-detect.ts

@@ -0,0 +1,43 @@
+import type { Builder, PluginRef, PostFixture } from './types.ts';
+
+export type DetectInput = {
+  theme: string;
+  plugins: PluginRef[];
+  posts: Pick<PostFixture, 'content' | 'meta'>[];
+};
+
+export function detectBuilder(input: DetectInput): Builder {
+  const active = new Set(input.plugins.filter((plugin) => plugin.active).map((plugin) => plugin.slug));
+  const theme = input.theme.toLowerCase();
+
+  if (
+    active.has('elementor') &&
+    input.posts.some((post) => post.meta._elementor_edit_mode !== undefined || post.meta._elementor_data !== undefined)
+  ) {
+    return 'elementor';
+  }
+
+  if (
+    (active.has('beaver-builder-lite-version') || active.has('bb-plugin')) &&
+    input.posts.some((post) => post.meta._fl_builder_data !== undefined || post.meta._fl_builder_enabled !== undefined)
+  ) {
+    return 'beaver';
+  }
+
+  if (
+    (theme === 'divi' || active.has('divi-builder')) &&
+    input.posts.some((post) => post.meta._et_pb_use_builder === 'on')
+  ) {
+    return 'divi';
+  }
+
+  if (input.posts.some((post) => post.content.includes('<!-- wp:'))) {
+    return 'gutenberg';
+  }
+
+  return 'classic';
+}
+
+export function siteInfoPayload(input: DetectInput): { builder: Builder } {
+  return { builder: detectBuilder(input) };
+}