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/form/app/fields/index.js

@@ -1,7 +0,0 @@
-import './form';
-import FileInstances from './file';
-import ArrayInstances from './array';
-import PageMedia, { Instance as PageMediaInstances } from './media';
-import './tabs';
-
-export default { FileInstances, ArrayInstances, Media: { PageMedia, PageMediaInstances } };