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/header.html.twig

@@ -1,11 +0,0 @@
-<nav id="nav">
-    <ul>
-        {% for page in pages %}
-            {% if page.visible %}
-            <li>
-                <a href="{{ page.url }}">{{ page.menu }}</a>
-            </li>
-            {% endif %}
-        {% endfor %}
-    </ul>
-</nav>