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/flex-objects/blueprints/flex-objects.yaml

@@ -1,16 +0,0 @@
-type: flex-objects
-
-form:
-  fields:
-
-    # Don't mess with this stuff below unless you know what you are doing
-    tools_section:
-      type: section
-      field_classes: overlay bottom
-
-      fields:
-        _post_entries_save:
-          label: PLUGIN_FLEX_OBJECTS.AFTER_SAVE
-          type: save-redirect
-          default: create-new
-