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/admin/vendor/autoload.php

@@ -1,25 +0,0 @@
-<?php
-
-// autoload.php @generated by Composer
-
-if (PHP_VERSION_ID < 50600) {
-    if (!headers_sent()) {
-        header('HTTP/1.1 500 Internal Server Error');
-    }
-    $err = 'Composer 2.3.0 dropped support for autoloading on PHP <5.6 and you are running '.PHP_VERSION.', please upgrade PHP or use Composer 2.2 LTS via "composer self-update --2.2". Aborting.'.PHP_EOL;
-    if (!ini_get('display_errors')) {
-        if (PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg') {
-            fwrite(STDERR, $err);
-        } elseif (!headers_sent()) {
-            echo $err;
-        }
-    }
-    trigger_error(
-        $err,
-        E_USER_ERROR
-    );
-}
-
-require_once __DIR__ . '/composer/autoload_real.php';
-
-return ComposerAutoloaderInit98c98c1c3d67f21a128f935fe4a74897::getLoader();