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-media-move/build.gradle

@@ -1,17 +0,0 @@
-plugins {
-    id("com.github.hierynomus.license").version("0.14.0")
-}
-apply plugin:'java'
-
-sourceSets {
-    grav {
-        resources {
-            srcDirs += "."
-            include  "**/*.yaml"
-            include  "**/*.php"
-            include  "**/*.css"
-            include  "**/*.js"
-            include  "**/*.twig"
-        }
-    }
-}