Sign in to other Elixir products with Elixir

A web product in the family can now use Elixir as its sign-in. It asks for one extra capability, account:email, and GET /oauth/userinfo answers the address the code already proved, so Elixir Clan and Elixir Drop know you are the same person you are here. The capability is the one exception to the consent page's ticked-by-default list: it appears only when an app names it, it can never be widened into from a checkbox or from Connections, and it is never advertised in a 401 challenge. Nothing changes for MCP clients and agents; no tool reads it.