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/error/templates/error.html.twig

@@ -1,3 +0,0 @@
-<h1>{{ 'PLUGIN_ERROR.ERROR'|t }} {{ header.http_response_code }}</h1>
-
-<p>{{ page.content|raw }}</p>