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/email/vendor/symfony/messenger/Handler/HandlersLocatorInterface.php

@@ -1,29 +0,0 @@
-<?php
-
-/*
- * This file is part of the Symfony package.
- *
- * (c) Fabien Potencier <fabien@symfony.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-namespace Symfony\Component\Messenger\Handler;
-
-use Symfony\Component\Messenger\Envelope;
-
-/**
- * Maps a message to a list of handlers.
- *
- * @author Nicolas Grekas <p@tchwork.com>
- */
-interface HandlersLocatorInterface
-{
-    /**
-     * Returns the handlers for the given message name.
-     *
-     * @return iterable<int, HandlerDescriptor>
-     */
-    public function getHandlers(Envelope $envelope): iterable;
-}