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.

api/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@wursor/api",
+  "private": true,
+  "type": "module",
+  "scripts": {}
+}