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/userinfo-avatar-credit.html.twig

@@ -1,10 +0,0 @@
-<p class="avatar">
-{{ "PLUGIN_ADMIN.AVATAR_BY"|t }}
-
-{% if config.system.accounts.avatar == 'gravatar' %}
-<a href="https://gravatar.com" target="_blank" rel="noopener noreferrer">gravatar.com</a>.
-{% else  %}
-<a href="https://multiavatar.com/" target="_blank" rel="noopener noreferrer">Multiavatar</a>.
-{% endif %}
-
-{{ "PLUGIN_ADMIN.AVATAR_UPLOAD_OWN"|t }}</p>