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/dashboard-notifications.html.twig

@@ -1,13 +0,0 @@
-<div id="notifications" class="dashboard-item admin-block default-box-shadow">
-    <h1>
-        {{ "PLUGIN_ADMIN.NOTIFICATIONS"|t }}
-        <span class="right">
-            <a href="#" class="button button-small" data-refresh="notifications"><i class="fa fa-refresh"></i></a>
-        </span>
-    </h1>
-
-    <div class="widget-content">
-        <div class="widget-loader"><i class="fa fa-refresh fa-spin"></i></div>
-        <ul></ul>
-    </div>
-</div>