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-legend.html.twig

@@ -1,11 +0,0 @@
-{% set legend = {'VISIBLE':'', 'NON_ROUTABLE':'not-routable', 'NON_VISIBLE':'not-visible', 'MODULE':'module'} %}
-<div id="pages-legend">
-    <strong>{{ 'PLUGIN_ADMIN.LEGEND'|t }}:</strong>
-    <ul>
-        {% for key, class in legend %}
-        <li><i class="page-icon fa fa-fw fa-circle-o {{ class }}"></i> {{ ('PLUGIN_ADMIN.'~key)|t }}</li>
-        {% endfor %}
-    </ul>
-</div>
-
-