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/modal-changes-detected.html.twig

@@ -1,13 +0,0 @@
-<div class="remodal" data-remodal-id="changes">
-    <form>
-        <h1>{{ "PLUGIN_ADMIN.MODAL_CHANGED_DETECTED_TITLE"|t }}</h1>
-        <p class="bigger">
-            {{ "PLUGIN_ADMIN.MODAL_CHANGED_DETECTED_DESC"|t }}
-        </p>
-        <br>
-        <div class="button-bar">
-            <a class="button secondary" data-leave-action="cancel" href="#"><i class="fa fa-fw fa-close"></i> {{ "PLUGIN_ADMIN.CANCEL"|t }}</a>
-            <a class="button" data-leave-action="continue" href="#"><i class="fa fa-fw fa-check"></i> {{ "PLUGIN_ADMIN.CONTINUE"|t }}</a>
-        </div>
-    </form>
-</div>