Add .plugin packaging (Cowork/Claude Code plugin variant)

plugin/ holds the plugin manifest, .mcp.json (CLAUDE_PLUGIN_ROOT paths), and
README; scripts/build-plugin.sh syncs the core client, esbuild-bundles the
server into a single ESM file (the plugin uploader rejects zip entries
containing "@", so no node_modules), and zips it as ms-todo.plugin. Same
server, two install formats (.mcpb + .plugin). Mirrors claude-msplanner.

Verified: the bundled server registers all 17 tools over stdio as ms-todo
v1.0.1.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugin/.claude-plugin/plugin.json

@@ -0,0 +1,10 @@
+{
+  "name": "ms-todo",
+  "version": "1.0.1",
+  "description": "Microsoft To Do connector — read and write your task lists, tasks, and checklist items via Microsoft Graph. Sign in once with todo_login.",
+  "author": {
+    "name": "PESCO Inc."
+  },
+  "license": "MIT",
+  "keywords": ["microsoft", "to do", "todo", "tasks", "graph", "productivity"]
+}