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/themes/grav/templates/partials/page-children.html.twig

@@ -1,11 +0,0 @@
-<ul class="grav-menu">
-    {% for page in pages.children %}
-    <li class="{{ (admin.route and page.route == context.route) ? 'Selected' : '' }} {{ page.children.count > 0 ? 'hasChildren' : '' }}">
-        <a href="{{ admin_route('/pages/' ~ page.route|trim('/')) }}"><em class="status {{ page.visible ? 'visible' : '' }}"></em>{{ page.menu }}</a>
-        {% if page.children.count > 0 %}
-        {% include 'partials/page-children.html.twig' with {pages: page} %}
-        {% endif %}
-    </li>
-    {% endfor %}
-</ul>
-