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/scss/nucleus.scss

@@ -1,27 +0,0 @@
-// REQUIRED DEPENDENCIES - DO NOT CHANGE
-
-// Load Third Party Libraries
-@import "vendor/bourbon/bourbon";
-
-// Load Nucleus Configuration
-@import "configuration/nucleus/base";
-
-// Load Nucleus Mixins and Functions
-@import "nucleus/functions/base";
-@import "nucleus/mixins/base";
-
-//-------------------------------------------
-
-// LOAD NUCLEUS COMPONENTS
-
-// Core
-@import "nucleus/core";
-
-// Flex
-@import "nucleus/flex";
-
-// Typography
-@import "nucleus/typography";
-
-// Forms
-@import "nucleus/forms";