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.

plugins/mmd-workspace/.claude-plugin/plugin.json

@@ -0,0 +1,6 @@
+{
+  "name": "mmd-workspace",
+  "version": "1.0.0",
+  "description": "MMD Group IT workspace skills: house-style documents and emails, the MMD Support ticket workflow, and safe data handling on the MMD AI gateway.",
+  "installationPreference": "required"
+}