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/twig/AdminTwigExtension.php

@@ -1,7 +0,0 @@
-<?php
-// Deprecated.
-
-require_once __DIR__ . '/../classes/plugin/Twig/AdminTwigExtension.php';
-
-user_error("require_once('user/plugins/admin/twig/AdminTwigExtension.php') is deprecated. All admin classes are using composer autoloader.", E_USER_DEPRECATED);
-class_alias('Grav\Plugin\Admin\Twig\AdminTwigExtension', 'Grav\Plugin\Admin\AdminTwigExtension');