Claude Code plugin manifest and marketplace install line

.claude-plugin/plugin.json makes the repository a single-skill plugin
(SKILL.md stays at the root). README shows the marketplace route next
to npx skills add; one route per machine.

.claude-plugin/plugin.json

@@ -0,0 +1,10 @@
+{
+  "name": "archify-vscode-ext",
+  "version": "1.0.0",
+  "description": "Author, validate, render and manage Archify diagrams and models (registries, decisions, C4 folders) as read by the Archify Diagram Viewer VS Code extension.",
+  "author": { "name": "Root At Skic" },
+  "license": "MIT",
+  "homepage": "https://gitea.lego-cloud.eu/vscode-extensions-skills/archify-vscode-ext",
+  "repository": "https://gitea.lego-cloud.eu/vscode-extensions-skills/archify-vscode-ext.git",
+  "keywords": ["archify", "architecture", "c4", "diagrams", "skills"]
+}