refactor: move delete/drop from Diagram to Table

Diagram is now purely a graph computation and inspection tool
(cascade, restrict, preview, prune). All mutation logic — transaction
management, SQL execution, prompts — lives in Table.delete() and
Table.drop(). Remove design docs superseded by datajoint-docs specs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko