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/error/blueprints.yaml

@@ -1,36 +0,0 @@
-name: Error
-version: 1.8.1
-description: Displays the error page.
-type: plugin
-slug: error
-icon: warning
-author:
-  name: Team Grav
-  email: devs@getgrav.org
-  url: http://getgrav.org
-homepage: https://github.com/getgrav/grav-plugin-error
-keywords: error, plugin, required
-bugs: https://github.com/getgrav/grav-plugin-error/issues
-license: MIT
-dependencies:
-  - { name: grav, version: '>=1.7.0' }
-
-form:
-  validation: strict
-  fields:
-    enabled:
-      type: toggle
-      label: PLUGIN_ADMIN.PLUGIN_STATUS
-      highlight: 1
-      default: 0
-      options:
-        1: PLUGIN_ADMIN.ENABLED
-        0: PLUGIN_ADMIN.DISABLED
-      validate:
-        type: bool
-
-    routes.404:
-      type: text
-      size: medium
-      label: PLUGIN_ERROR.ROUTE_404
-      default: '/error'