chore: init wp-multi-city v0.1.0 with S1 skeleton + S2 design spec

S1 skeleton (cp-8ci, already done):
- wp-multi-city.php with header, constants, hooks bootstrap
- includes/class-plugin.php with PSR-4-ish autoloader, ACF dependency guard
- includes/class-activator.php placeholder
- composer.json (PHP 8.0, phpunit dev)
- README.md with roadmap
- uninstall.php placeholder

S2 design (cp-q7g):
- docs/superpowers/specs/2026-05-12-s2-taxonomy-cpt-acf-design.md

includes/class-activator.php

@@ -0,0 +1,18 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity;
+
+final class Activator
+{
+    public static function activate(): void
+    {
+        // Stage 2 will register taxonomy + CPT; flush after they're declared.
+        flush_rewrite_rules();
+    }
+
+    public static function deactivate(): void
+    {
+        flush_rewrite_rules();
+    }
+}