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/installed.php

@@ -1,23 +0,0 @@
-<?php return array(
-    'root' => array(
-        'pretty_version' => 'dev-develop',
-        'version' => 'dev-develop',
-        'type' => 'grav-plugin',
-        'install_path' => __DIR__ . '/../../',
-        'aliases' => array(),
-        'reference' => 'dfa9760987bbf2cdd61038bca5a7928b31a9dcff',
-        'name' => 'getgrav/grav-plugin-flex-objects',
-        'dev' => false,
-    ),
-    'versions' => array(
-        'getgrav/grav-plugin-flex-objects' => array(
-            'pretty_version' => 'dev-develop',
-            'version' => 'dev-develop',
-            'type' => 'grav-plugin',
-            'install_path' => __DIR__ . '/../../',
-            'aliases' => array(),
-            'reference' => 'dfa9760987bbf2cdd61038bca5a7928b31a9dcff',
-            'dev_requirement' => false,
-        ),
-    ),
-);