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/mailer/Header/MetadataHeader.php

@@ -1,34 +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\Mailer\Header;
-
-use Symfony\Component\Mime\Header\UnstructuredHeader;
-
-/**
- * @author Kevin Bond <kevinbond@gmail.com>
- */
-final class MetadataHeader extends UnstructuredHeader
-{
-    private $key;
-
-    public function __construct(string $key, string $value)
-    {
-        $this->key = $key;
-
-        parent::__construct('X-Metadata-'.$key, $value);
-    }
-
-    public function getKey(): string
-    {
-        return $this->key;
-    }
-}