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/PluginBootS7Test.php

@@ -41,6 +41,9 @@ final class PluginBootS7Test extends TestCase
         Plugin::instance()->boot();
         Plugin::instance()->boot();
 
-        self::assertTrue(true, 'Double boot must not throw');
+        self::assertTrue(
+            class_exists('YahnisElsts\\PluginUpdateChecker\\v5\\PucFactory'),
+            'PucFactory must still be loaded after double boot()'
+        );
     }
 }