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/flex-objects/flex-objects.yaml

@@ -1,14 +0,0 @@
-enabled: true
-
-built_in_css: true
-extra_admin_twig_path: 'theme://admin/templates'
-admin_list:
-  per_page: 15
-  order:
-    by: updated_timestamp
-    dir: desc
-
-directories:
-  - 'blueprints://flex-objects/pages.yaml'
-  - 'blueprints://flex-objects/user-accounts.yaml'
-  - 'blueprints://flex-objects/user-groups.yaml'