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/.github/FUNDING.yml

@@ -1,8 +0,0 @@
-# These are supported funding model platforms
-
-github: # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
-patreon: # Replace with a single Patreon username
-open_collective: grav
-ko_fi: # Replace with a single Ko-fi username
-tidelift: # Replace with a single Tidelift platform-name/package-name e.g., npm/babel
-custom: # Replace with a single custom sponsorship URL

plugins/admin/.github/dependabot.yml

@@ -1,6 +0,0 @@
-version: 2
-updates:
-- package-ecosystem: "github-actions"
-  directory: "/"
-  schedule:
-      interval: "weekly"