refactor(wpmc S7): tighten tests + cleanup bootstrap shims

- PluginBootS7Test idempotency: assertTrue(class_exists) instead of assertTrue(true)
- ZipArchiveBuildTest: try/finally cleanup + unzip exit-code check
- bootstrap.php: drop unused PUC shims (wp_remote_post, transient*)
- CHANGELOG.md: header references Keep a Changelog (was: mirrors readme.txt)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/bootstrap.php

@@ -68,21 +68,6 @@ if (!function_exists('update_option')) {
         return true;
     }
 }
-if (!function_exists('set_transient')) {
-    function set_transient(string $key, $value, int $expiry = 0): bool {
-        return true;
-    }
-}
-if (!function_exists('get_transient')) {
-    function get_transient(string $key) {
-        return false;
-    }
-}
-if (!function_exists('delete_transient')) {
-    function delete_transient(string $key): bool {
-        return true;
-    }
-}
 if (!function_exists('wp_next_scheduled')) {
     function wp_next_scheduled(string $hook, array $args = []) {
         return false;
@@ -114,12 +99,6 @@ if (!function_exists('wp_remote_get')) {
         return false;
     }
 }
-if (!function_exists('wp_remote_post')) {
-    function wp_remote_post(string $url, array $args = []) {
-        return false;
-    }
-}
-
 require_once __DIR__ . '/../includes/class-plugin.php';
 \WPMultiCity\Plugin::register_autoloader();