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/css/nucleus-ie9.css

@@ -1,62 +0,0 @@
-/* IE9 Resets and Normalization */
-article,
-aside,
-details,
-figcaption,
-figure,
-footer,
-header,
-hgroup,
-main,
-nav,
-section,
-summary {
-	display: block;
-}
-
-audio,
-canvas,
-progress,
-video {
-	display: inline-block;
-}
-
-[hidden],
-template {
-	display: none;
-}
-
-abbr[title] {
-	border-bottom: 1px dotted;
-}
-
-img {
-	border: 0;
-}
-
-svg:not(:root) {
-	overflow: hidden;
-}
-
-figure {
-	margin: 1em 40px;
-}
-
-button {
-	overflow: visible;
-}
-
-input[type="checkbox"],
-input[type="radio"] {
-	box-sizing: border-box;
-	padding: 0;
-}
-
-legend {
-	border: 0;
-	padding: 0;
-}
-
-textarea {
-	overflow: auto;
-}