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/flex-objects/templates/flex-objects.html.twig

@@ -1,21 +0,0 @@
-{% extends 'partials/base.html.twig' %}
-
-{% block content %}
-    {% block page_content %}
-        {{ page.content|raw }}
-    {% endblock %}
-
-    {% block flex_content %}
-        {% set options = page.header.flex %}
-        {% set page_assets = page.header.assets %}
-        {% set type = options.directory ?? page.header.directory %}
-        {% if type %}
-            {% set list = type|array %}
-        {% else %}
-            {% set type = uri.param('directory') %}
-        {% endif %}
-        {% set id = options.id ?? uri.param('id') %}
-
-        {% include 'flex-objects/page.html.twig' %}
-    {% endblock %}
-{% endblock %}