Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/admin/classes/plugin/Themes.php

@@ -1,29 +0,0 @@
-<?php
-
-/**
- * @package    Grav\Plugin\Admin
- *
- * @copyright  Copyright (c) 2015 - 2024 Trilby Media, LLC. All rights reserved.
- * @license    MIT License; see LICENSE file for details.
- */
-
-namespace Grav\Plugin\Admin;
-
-/**
- * Admin theme object
- *
- * @author  RocketTheme
- * @license MIT
- */
-class Themes extends \Grav\Common\Themes
-{
-    public function init()
-    {
-        /** @var Themes $themes */
-        $themes = $this->grav['themes'];
-        $themes->configure();
-        $themes->initTheme();
-
-        $this->grav->fireEvent('onAdminThemeInitialized');
-    }
-}