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

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+{
+  "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"]
+}

plugin/.mcp.json

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+{
+  "mcpServers": {
+    "ms-todo": {
+      "command": "node",
+      "args": ["${CLAUDE_PLUGIN_ROOT}/server/index.mjs"],
+      "env": {
+        "TODO_CLIENT_ID": "9af4a8a3-5290-4089-9058-a29dcce63c4f",
+        "TODO_TENANT_ID": "94c6c62d-8fe7-416f-8fef-7d8620b95819"
+      }
+    }
+  }
+}

plugin/README.md

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+# 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.