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/langswitcher/vendor/autoload.php

@@ -1,7 +0,0 @@
-<?php
-
-// autoload.php @generated by Composer
-
-require_once __DIR__ . '/composer/autoload_real.php';
-
-return ComposerAutoloaderInitc810fb4e9d827a69b99d6583ac14140c::getLoader();