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/.mcp.json

@@ -0,0 +1,12 @@
+{
+  "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"
+      }
+    }
+  }
+}