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.

plugin/composer.json

@@ -0,0 +1,13 @@
+{
+  "name": "wursor/plugin",
+  "description": "Wursor WordPress connector",
+  "type": "wordpress-plugin",
+  "license": "GPL-2.0-or-later",
+  "require": {
+    "php": ">=7.4"
+  },
+  "require-dev": {
+    "phpunit/phpunit": "^9.6",
+    "yoast/phpunit-polyfills": "^2.0"
+  }
+}