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/automagic-images/automagic-images.yaml

@@ -1,27 +0,0 @@
-enabled: '1'
-adapter: imagick
-remove_original: '0'
-sizes:
-  -
-    width: 640
-    quality: 92
-  -
-    width: 1000
-    quality: 90
-  -
-    width: 1500
-    quality: 87
-  -
-    width: 2500
-    quality: 85
-  -
-    width: 3500
-    quality: 82
-
-sizesattr:
-  -
-    class: 'default'
-    directive: '100vw'
-  -
-    class: 'example'
-    directive: '(min-width: 20em) 33%, 100vw'