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/load-site.ts

@@ -0,0 +1,11 @@
+import { readFileSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import type { SiteFixture } from './types.ts';
+
+const goldenRoot = join(dirname(fileURLToPath(import.meta.url)), '..');
+
+export function loadSite(id: string): SiteFixture {
+  const raw = readFileSync(join(goldenRoot, 'sites', id, 'site.json'), 'utf8');
+  return JSON.parse(raw) as SiteFixture;
+}