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/flex-objects/vendor/composer/autoload_classmap.php

@@ -1,11 +0,0 @@
-<?php
-
-// autoload_classmap.php @generated by Composer
-
-$vendorDir = dirname(dirname(__FILE__));
-$baseDir = dirname($vendorDir);
-
-return array(
-    'Composer\\InstalledVersions' => $vendorDir . '/composer/InstalledVersions.php',
-    'Grav\\Plugin\\FlexObjectsPlugin' => $baseDir . '/flex-objects.php',
-);