feat: add mmd-it marketplace + mmd-workspace plugin (4 MMD skills)

Org-wide Claude Desktop plugin marketplace for MMD Group IT. Adds the
mmd-it marketplace manifest and the mmd-workspace plugin
(installationPreference: required) carrying four skills grounded in
real MMD material: mmd-document-style, mmd-email-style,
mmd-it-support, mmd-data-handling. Public repo, no secrets — MCP
credentials are delivered separately via managedMcpServers.

.claude-plugin/marketplace.json

@@ -0,0 +1,13 @@
+{
+  "name": "mmd-it",
+  "owner": {
+    "name": "MMD Group IT"
+  },
+  "plugins": [
+    {
+      "name": "mmd-workspace",
+      "source": "./plugins/mmd-workspace",
+      "description": "MMD Group IT workspace skills for Claude Desktop: house-style documents and emails, the MMD Support ticket workflow, and safe data handling on the MMD AI gateway."
+    }
+  ]
+}