Update oh-my-design

Update only the OmD installation the user already has. Preserve its project/global scope, installed channels, Cursor compatibility mode, DESIGN.md, references added by the user, and every file outside OmD ownership.

Workflow

1. Run from the intended project root unless the user explicitly requests the global installation.

2. Use the latest package as the executor:

   npx oh-my-design-cli@latest update