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/problems/css/admin.min.css

@@ -1 +0,0 @@
-#admin-main .admin-block .report-output h1{margin-top:2rem}#admin-main .admin-block .report-output .toast .btn{font-size:90%;float:right;margin-top:-2px;margin-right:.5rem;padding:2px 7px;border-radius:3px}.report-output ul.problems{margin:1rem 0;padding:0;list-style:none}.report-output ul.problems h5{margin:0}.report-output ul.problems li.menu{margin-bottom:1rem;box-shadow:0 10px 20px -10px rgba(0,0,0,.2)}.report-output ul.problems .toast{font-size:1rem;padding:.5rem 1.5rem}.report-output ul.problems .toast .btn{margin-left:1rem;text-decoration:none!important}.report-output ul.problems .toast .btn i{margin-right:.3rem}.report-output ul.problems ul.details{padding-bottom:1rem;padding-left:0;list-style:none}.report-output ul.problems ul.details li{padding-right:1.5rem;padding-left:1.5rem}.report-output ul.problems ul.details code{font-size:90%;line-height:1.2;padding:.1rem .2rem;vertical-align:middle;border-radius:.1rem}.report-output ul.problems ul.details .menu-item{margin-top:.5rem!important;padding-top:.5rem}.report-output ul.problems ul.details .menu-item:first-child{border:none}.report-output ul.problems ul.details .menu-badge{display:inline;float:right;padding:0}.report-output ul.problems ul.details .menu-badge .label{font-size:1rem;line-height:1.5rem;width:1.5rem;height:1.5rem;border-radius:3px}