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/login-logout.html.twig

@@ -1,15 +0,0 @@
-{% embed 'partials/login.html.twig' with {title:'Grav Admin Logout'} %}
-
-{% block integration %}
-{% endblock %}
-
-{% block form %}
-
-    <div class="form-actions primary-accent">
-        <a class="button secondary" onclick="window.history.back()"><i class="fa fa-reply"></i> {{ 'PLUGIN_ADMIN.BACK'|t }}</a>
-        <button type="submit" class="button primary" name="task" value="logout"><i class="fa fa-sign-in"></i> {{ 'PLUGIN_ADMIN.LOGOUT'|t }}</button>
-    </div>
-
-{% endblock %}
-
-{% endembed %}