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/mime/Encoder/Base64Encoder.php

@@ -1,41 +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\Mime\Encoder;
-
-/**
- * @author Chris Corbyn
- */
-class Base64Encoder implements EncoderInterface
-{
-    /**
-     * Takes an unencoded string and produces a Base64 encoded string from it.
-     *
-     * Base64 encoded strings have a maximum line length of 76 characters.
-     * If the first line needs to be shorter, indicate the difference with
-     * $firstLineOffset.
-     */
-    public function encodeString(string $string, ?string $charset = 'utf-8', int $firstLineOffset = 0, int $maxLineLength = 0): string
-    {
-        if (0 >= $maxLineLength || 76 < $maxLineLength) {
-            $maxLineLength = 76;
-        }
-
-        $encodedString = base64_encode($string);
-        $firstLine = '';
-        if (0 !== $firstLineOffset) {
-            $firstLine = substr($encodedString, 0, $maxLineLength - $firstLineOffset)."\r\n";
-            $encodedString = substr($encodedString, $maxLineLength - $firstLineOffset);
-        }
-
-        return $firstLine.trim(chunk_split($encodedString, $maxLineLength, "\r\n"));
-    }
-}