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/form/templates/forms/layouts/button/default-button.html.twig

@@ -1,12 +0,0 @@
-{% set button_tag %}
-<button
-    {% block embed_button_core %}{% endblock %}
-    {% block embed_button_classes %}{% endblock %}
->{%- block embed_button_content -%}{%- endblock -%}</button>
-{% endset %}
-
-{% if button_url %}
-  <a href="{{ button_url|e }}">{{ button_tag|trim|raw }}</a>
-{% else %}
-  {{ button_tag|trim|raw }}
-{% endif %}