Add a select op that sets the document selection (by hex handles from
query, or by type/layer, or clear). Selection drives modify commands, so
an agent can query → select → run("ERASE") to act on specific entities.
Completes the modify half of the act/observe loop. ocs.py gains select().
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>