A connection carries the account that signed it in

[czpython]

The Connection handle exposes id, scopes, identity, and connected_at, but not the druks account that owns the sign-in. Extensions that need the owner had to reach into connection.row.

• Connection.account_id reads the owner off the row.
• The Connections section of writing-an-extension.md lists the new field.
• The handle test asserts the owner comes back.