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/automagic-images/adapters/interface.php

@@ -1,15 +0,0 @@
-<?php
-namespace Grav\Plugin;
-
-interface ResizeAdapterInterface
-{
-    public function __construct($source);
-
-    public function resize($width, $height);
-
-    public function getFormat();
-
-    public function setQuality($quality);
-
-    public function save($filename);
-}