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/README.md

@@ -0,0 +1,36 @@
+# Microsoft To Do plugin
+
+Connects Claude to Microsoft To Do via the Microsoft Graph API. This is the
+plugin (`.plugin`) packaging of the same MCP stdio server shipped as the
+`ms-todo.mcpb` Claude Desktop extension — one codebase, two install formats.
+
+## Install
+
+Upload `ms-todo.plugin` in the plugin picker. Then ask Claude to run
+`todo_login` once — a browser window opens for Microsoft sign-in. The refresh
+token is cached at `~/.ms-todo/token-cache.json` (shared with the
+Desktop-extension install, so you only ever sign in once per machine).
+
+## Requirements
+
+- Node.js >= 18 on PATH (the server is a Node stdio process).
+- A Microsoft work account in the PESCO tenant. Auth is delegated
+  (auth-code + PKCE); scopes: Tasks.ReadWrite, User.Read.
+
+## Configuration
+
+Environment is baked into `.mcp.json` (client + tenant id). Optional overrides:
+
+| Variable           | Purpose                                                  |
+| ------------------ | -------------------------------------------------------- |
+| `TODO_TOKEN_CACHE` | Alternate token cache path                               |
+| `TODO_TIMEZONE`    | Zone for due/start/reminder dates (default UTC)          |
+| `TODO_READONLY`    | "true" registers only the viewing tools                  |
+
+## Tools
+
+17 tools: `todo_login/logout/test`, list CRUD (`todo_list_lists/create_list/
+update_list/delete_list`), task CRUD (`todo_list_tasks/get_task/create_task/
+update_task/complete_task/delete_task`), checklist items (`todo_list_checklist/
+add_checklist_item/check_checklist_item/delete_checklist_item`).
+See the repository README for the full list.